Q.The value of ∫−ππsin3xcos2xdx is _______.
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Definite Integral Symmetry: The Shortcut That Saves You Work
Asked to find the area under f(x)=x3 from x=−2 to x=2? You could integrate directly — but there's a much faster way if you notice the symmetry of the graph.
The Intuition: What Does "Symmetry" Mean Here?
A function can be symmetric about the y-axis (like x2 or cosx) or about the origin (like x3 or sinx). When you integrate over a symmetric interval — from −a to a — these symmetries create a predictable cancellation or doubling.
Even functions (symmetric about the y-axis): f(−x)=f(x) — think x2, x4, cosx, ∣x∣. The left side mirrors the right, so the area from −a to 0 equals the area from 0 to a. The total is double the area on one side.
Odd functions (symmetric about the origin): f(−x)=−f(x) — think x3, x5, sinx, tanx. The left side is the negative mirror of the right, so every positive area on the right is cancelled by an equal negative area on the left. The total is zero.
This only works when the limits are symmetric about zero — from −a to a. For [0,a] or [1,3], symmetry doesn't help directly.
The Precise Statement
Let f be continuous on [−a,a].
- If f is even (f(−x)=f(x)), then ∫−aaf(x)dx=2∫0af(x)dx
- If f is odd (f(−x)=−f(x)), then ∫−aaf(x)dx=0
∫−aaf(x)dx={2∫0af(x)dx0if f is evenif f is odd
Why Does This Work? (A Quick Proof)
Split at zero:
∫−aaf(x)dx=∫−a0f(x)dx+∫0af(x)dx
For the first term substitute u=−x (dx=−du; x=−a→u=a, x=0→u=0):
∫−a0f(x)dx=∫0af(−u)du
Now use symmetry:
- If f is even, f(−u)=f(u), so this becomes ∫0af(u)du; adding the second term gives 2∫0af(x)dx.
- If f is odd, f(−u)=−f(u), so this becomes −∫0af(u)du; adding the second term gives 0.
Common Mistakes to Avoid
Don't assume symmetry without checking. A "balanced"-looking function need not be even or odd — e.g. f(x)=x2+x is neither, so these formulas don't apply. Always verify f(−x)=f(x) or f(−x)=−f(x) for all x.
The interval must be [−a,a]. If the limits are [−2,3], symmetry doesn't apply directly — split the integral or shift variables.
Examples to Cement the Idea
Example 1: ∫−33x4dx — x4 is even, so
∫−33x4dx=2∫03x4dx=2[5x5]03=2⋅5243=5486 …
The key idea is Definite Integral Symmetry: for an odd function f(x), ∫−aaf(x)dx=0.
Step 1: Let f(x)=sin3xcos2x. Check parity:
sin3(−x)=−sin3x and cos2(−x)=cos2x, so f(−x)=(−sin3x)(cos2x)=−f(x). Hence f(x) is odd. …
Using the property that the integral of an odd function over a symmetric interval [−a,a] is zero, we identify the integrand sin3xcos2x as an odd function. Hence the integral evaluates to 0.
The key to solving this integral lies in recognising symmetry — specifically, whether the function you're integrating is odd or even over the interval [−π,π].
A function f(x) is odd if f(−x)=−f(x) for all x in its domain. For an odd function, the definite integral over a symmetric interval [−a,a] is always zero:
∫−aaf(x)dx=0
This is because the area on the left side of the y-axis exactly cancels the area on the right side.
Now, look at our integrand: sin3x⋅cos2x. We need to check if it's odd.
-
Check the parity of each factor.
- sin3x is an odd function: sin(−3x)=−sin3x.
- cos2x is an even function: cos(−2x)=cos2x.
-
Combine them.
The product of an odd function and an even function is odd. Let's verify:
Let f(x)=sin3xcos2x. Then
f(−x)=sin(−3x)cos(−2x)=(−sin3x)(cos2x)=−sin3xcos2x=−f(x)
So indeed f(x) is odd.
- Apply the symmetry property. Since the interval [−π,π] is symmetric about zero and f(x) is odd, we have: …
Method: Parity test before integrating over [−a,a]
Before doing any work on ∫−aaf(x)dx, test the integrand's parity — an odd integrand integrates to 0 instantly.
Steps
Step 1: Compute f(−x).
If f(−x)=−f(x) the function is odd; if f(−x)=f(x) it is even.
Step 2: Use the product parity rules. …
Common Mistakes
Mistake 1: Mis-identifying the parity of sin3xcos2x.
Why it's wrong: odd (sin) × even (cos) = odd, so f(−x)=−f(x); treating it as even leads you to expect a nonzero value. Correct approach: verify f(−x)=−sin3xcos2x and conclude the integral is 0.
Mistake 2: Grinding through product-to-sum instead of using symmetry. …
Showing the 12 most recent of 48 on this concept.
- AP EAPCET 2022Set eng-2022-07-05-AN1 markMCQQ.∫−ππ1+cos2x2x(1+sinx)dx= (A) 2π (B) π2 (C) π+2 (D) π/2
›Reveal solutionSolution
Splitting the integrand by parity kills the odd piece immediately; the remaining even piece is handled with the classic "∫0πxf(sinx,cosx)dx=2π∫0πf(sinx,cosx)dx"-style symmetry trick, collapsing to a simple arctangent integral.
Concept and Intuition
Whenever a definite integral is over a symmetric interval like [−π,π], always check parity first: odd integrands vanish, and even integrands can be doubled and computed over [0,π] only. For integrals of the form ∫0πxg(sinx,cosx)dx, the substitution x→π−x often converts the "x" factor into "π−x", letting you solve for the integral in terms of a simpler one without the x multiplier.
Step-by-Step Solution
- Write 1+cos2x2x(1+sinx)=1+cos2x2x+1+cos2x2xsinx.
- The first term, 1+cos2x2x, is odd (odd/even = odd), so ∫−ππ of it is 0.
- The second term, 1+cos2x2xsinx, is even (odd×odd = even, divided by even), so
∫−ππ1+cos2x2xsinxdx=2∫0π1+cos2x2xsinxdx=4∫0π1+cos2xxsinxdx.
- Let J=∫0π1+cos2xxsinxdx. Substituting x→π−x (using sin(π−x)=sinx, cos(π−x)=−cosx, and cos2 unchanged): J=∫0π1+cos2x(π−x)sinxdx=π∫0π1+cos2xsinxdx−J. …
- AP EAPCET 2023Set eng-2023-05-18-FN1 markMCQQ.∫0πxsin3xcos2xdx= (A) 152π (B) 154π (C) 30π (D) 52π
›Reveal solutionSolution
Using the symmetry property ∫0πxg(x)dx=2π∫0πg(x)dx (valid since g(π−x)=g(x) here) reduces the problem to a simple substitution integral, giving 152π.
Concept and Intuition
Whenever the integrand has the form x⋅g(x) over [0,π] and g(π−x)=g(x) (i.e., g is symmetric about x=π/2), we can use the standard trick: let I=∫0πxg(x)dx, substitute x→π−x to get I=∫0π(π−x)g(x)dx=π∫0πg(x)dx−I, so 2I=π∫0πg(x)dx, i.e. I=2π∫0πg(x)dx. This avoids ever integrating xsin3xcos2x directly by parts.
Step-by-Step Solution
- Let g(x)=sin3xcos2x. Check symmetry: g(π−x)=sin3(π−x)cos2(π−x)=(sinx)3(−cosx)2=sin3xcos2x=g(x). ✓
- So ∫0πxsin3xcos2xdx=2π∫0πsin3xcos2xdx.
- Compute ∫0πsin3xcos2xdx=∫0πsinx(1−cos2x)cos2xdx. Substitute u=cosx, du=−sinxdx; limits x:0→π give u:1→−1.
- This becomes ∫1−1(1−u2)u2(−du)=∫−11(u2−u4)du. …
- AP EAPCET 2024Set eng-2024-05-22-FN1 markMCQQ.∫−ππ1+cos2xxsinxdx= (A) 43π2 (B) 2π+1 (C) 4π2 (D) 2π2
›Reveal solutionSolution
Combine the even-function property over [−π,π] with the classic x→π−x symmetry trick for integrals of xg(sinx,cosx). Answer: π2/2.
Concept and Intuition
First check parity: since sin(−x)=−sinx and cos2(−x)=cos2x, the integrand f(x)=1+cos2xxsinx satisfies f(−x)=f(x) — it's even, so the integral over [−π,π] is twice the integral over [0,π]. Then, for integrals of x times a function of sinx,cosx over [0,π], the substitution x→π−x is the standard tool to eliminate the explicit x.
Step-by-Step Solution
- Since f(−x)=f(x): ∫−ππfdx=2∫0πfdx=2J, where J=∫0π1+cos2xxsinxdx.
- Substitute x→π−x in J: sin(π−x)=sinx, cos(π−x)=−cosx (so cos2 unchanged): J=∫0π1+cos2x(π−x)sinxdx=π∫0π1+cos2xsinxdx−J.
- So 2J=πK where K=∫0π1+cos2xsinxdx. …
- AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQQ.∫03π/2cos3x+sin3xcos3xdx= (A) 0 (B) 1 (C) 4π (D) 43π
›Reveal solutionSolution
Using the substitution x→23π−x swaps the roles of cos3x and sin3x in the integrand, showing the given integral equals its "sine" counterpart; since the two together give the full interval length, each equals 43π.
Concept and Intuition
This is a King's-rule-style symmetry trick: ∫0af(x)dx=∫0af(a−x)dx. Applying it with a=3π/2 swaps cosx↔−sinx and sinx↔−cosx, which exactly interchanges the roles of cos3x and sin3x in the fraction (the minus signs cancel through the cube and the overall ratio), letting us equate the two "complementary" integrals.
Step-by-Step Solution
- Define I=∫03π/2cos3x+sin3xcos3xdx and J=∫03π/2cos3x+sin3xsin3xdx.
- Adding: I+J=∫03π/21dx=23π.
- Substitute x→23π−x in I: cos(23π−x)=−sinx and sin(23π−x)=−cosx. …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.∫−2π2π(1+cosx)3(1−cosx)4dx= (A) 0 (B) 5π (C) 25π (D) 45π
›Reveal solutionSolution
Factoring (1+cosx)3(1−cosx)4 into sin6x(1−cosx) and splitting the integral shows the cosxsin6x part vanishes by symmetry, leaving 4∫0πsin6xdx=45π.
Concept and Intuition
Products of (1±cosx) raised to powers usually simplify via (1+cosx)(1−cosx)=1−cos2x=sin2x. Pairing three factors from each gives sin6x, leaving one leftover (1−cosx) factor. Splitting that leftover separates the integral into an even, periodic sin6x piece and an odd-derivative sin6xcosx piece that integrates to zero over a range where sinx returns to the same value at both ends.
Step-by-Step Solution
- Rewrite the product:
(1+cosx)3(1−cosx)4=[(1+cosx)(1−cosx)]3⋅(1−cosx)=(sin2x)3(1−cosx)=sin6x−sin6xcosx
- Split the integral:
∫−2π2πsin6xdx−∫−2π2πsin6xcosxdx
- Second integral: sin6xcosx=dxd(7sin7x), so
∫−2π2πsin6xcosxdx=[7sin7x]−2π2π=70−0=0
since sin(2π)=sin(−2π)=0.
4. First integral: sin6x has period π (because sin(x+π)=−sinx⇒sin6(x+π)=sin6x). The interval [−2π,2π] has length 4π, i.e. exactly 4 periods of length π:
∫−2π2πsin6xdx=4∫0πsin6xdx …
- AP EAPCET 2026Set eng-2026-05-13-AN1 markMCQQ.∫−3π/2−π/2((x+π)3+cos2(x+3π))dx= (A) 8π (B) 2π (C) 4π−1 (D) 32π4
›Reveal solutionSolution
Shifting the variable to u=x+π turns the limits into a symmetric interval about zero; the odd cubic term vanishes and only the even cos2u term survives, giving π/2.
Concept and Intuition
Whenever an integral's limits and integrand both have a shift-symmetry, substituting to center the interval at 0 lets us exploit odd/even function properties: odd functions integrate to zero over [−a,a], and even functions can be doubled over [0,a].
Step-by-Step Solution
- Let u=x+π, so du=dx. When x=−3π/2, u=−π/2; when x=−π/2, u=π/2.
- cos2(x+3π)=cos2((x+π)+2π)=cos2(u+2π)=cos2u (cosine has period 2π).
- The integral becomes ∫−π/2π/2(u3+cos2u)du.
- u3 is an odd function, so ∫−π/2π/2u3du=0.
- cos2u is even, so ∫−π/2π/2cos2udu=2∫0π/2cos2udu. …
- AP EAPCET 2026Set eng-2026-05-15-AN1 markMCQQ.∫−ππ1+axcos2xdx, (a>0) = (A) aπ (B) aπ (C) 2π (D) 2π
›Reveal solutionSolution
This is the classic "King's rule" property ∫−aa1+bxf(x)dx=21∫−aaf(x)dx for even f; here it directly gives I=π/2, independent of a.
Concept and Intuition
Whenever an even function f(x) is divided by 1+bx and integrated over a symmetric interval [−a,a], replacing x→−x swaps 1/(1+bx) with bx/(1+bx) — and since these two fractions add to exactly 1, averaging the original and transformed integral removes the exponential entirely, leaving half of ∫−aaf(x)dx. This makes the ax term a red herring: the answer depends only on cos2x.
Step-by-Step Solution
- Let I=∫−ππ1+axcos2xdx.
- Substitute x→−x (valid since limits are symmetric): I=∫−ππ1+a−xcos2(−x)dx=∫−ππ1+a−xcos2xdx (using cos2(−x)=cos2x).
- Simplify 1+a−x1=ax+1ax, so I=∫−ππ1+axcos2x⋅axdx.
- Add the two expressions for I: 2I=∫−ππcos2x[1+ax1+1+axax]dx=∫−ππcos2xdx. …
- AP EAPCET 2024Set eng-2024-05-20-FN1 markMCQQ.∫−ππ4−cos2xxsin3xdx= (A) 2π(1−log3) (B) 2π(1−43log3) (C) π(1−43log3) (D) 4π(1−log3)
›Reveal solutionSolution
Use symmetry (odd integrand times x, plus f(π−x)=f(x)) to strip the x out of the integral, then finish with a u=cosx substitution and partial fractions.
Concept and Intuition
Integrals of the form ∫−aaxg(x)dx where g is odd become 2∫0axg(x)dx (since xg(x) is even). If additionally g(π−x)=g(x) on [0,π], the classic "King's Rule" trick ∫0πxg(x)dx=2π∫0πg(x)dx removes the x factor entirely.
Step-by-Step Solution
- Let g(x)=4−cos2xsin3x. Since sin3(−x)=−sin3x and cos2(−x)=cos2x, g is odd, so xg(x) is even: ∫−ππxg(x)dx=2∫0πxg(x)dx.
- Also g(π−x)=4−cos2(π−x)sin3(π−x)=4−cos2xsin3x=g(x) (since sin(π−x)=sinx, cos(π−x)=−cosx), so King's Rule gives ∫0πxg(x)dx=2π∫0πg(x)dx.
- Combining: original integral =2⋅2π∫0πg(x)dx=π∫0πg(x)dx=πJ.
- Compute J=∫0π4−cos2xsin3xdx via u=cosx: J=∫−114−u21−u2du=2∫01[1−4−u23]du (using 1−u2=(4−u2)−3). …
- AP EAPCET 2024Set eng-2024-05-20-AN1 markMCQQ.∫0π1+cos2xxsinxdx= (A) 0 (B) 2π (C) 2π2 (D) 4π2
›Reveal solutionSolution
This is the classic "xf(sinx)" King's-rule setup; it reduces to π/2 times a standard arctan integral, giving 4π2 — (D).
Concept and Intuition
Whenever the integrand has the shape x⋅g(sinx,cos2x) over [0,π] and g is invariant under x→π−x (true here since sin(π−x)=sinx and cos2(π−x)=cos2x), the King's-rule substitution x→π−x lets us replace the "x" prefactor with "π−x", and adding the two forms eliminates the awkward x entirely.
Step-by-Step Solution
- Let I=∫0π1+cos2xxsinxdx and f(x)=1+cos2xsinx.
- Check f(π−x)=1+cos2(π−x)sin(π−x)=1+cos2xsinx=f(x) — confirmed invariant.
- By King's rule: I=∫0π(π−x)f(x)dx=π∫0πf(x)dx−I.
- So 2I=π∫0πf(x)dx=πJ, where J=∫0π1+cos2xsinxdx. …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.∫−π/2π/2sin2xcos2x(sinx+cosx)dx= (A) 0 (B) 152 (C) 154 (D) 52
›Reveal solutionSolution
This tests using odd/even symmetry over a symmetric interval to kill half the integral, then a simple u=sinx substitution for the rest. The answer is 154, option (C).
Concept and Intuition
Over a symmetric interval [−a,a], an odd integrand integrates to zero and an even integrand integrates to twice the integral over [0,a]. Expanding (sinx+cosx) splits the problem cleanly into one odd term and one even term, so only the even term survives — turning a seemingly complicated integral into a single elementary substitution.
Step-by-Step Solution
- Expand: sin2xcos2x(sinx+cosx)=sin3xcos2x+sin2xcos3x.
- g(x)=sin3xcos2x: since sin3(−x)=−sin3x and cos2(−x)=cos2x, g(−x)=−g(x) — odd. So ∫−π/2π/2g(x)dx=0.
- h(x)=sin2xcos3x: both factors are even, so h is even, and ∫−π/2π/2hdx=2∫0π/2hdx.
- Write h(x)=sin2xcos2xcosx=sin2x(1−sin2x)cosx. Let u=sinx, du=cosxdx; limits 0→1. …
- AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQQ.∫−2π2πsin4(2x)cos6(2x)dx= (A) 643π (B) 649π (C) 359π (D) 2809π
›Reveal solutionSolution
Substitute u=2x, use periodicity (period π) to reduce to 8 copies of a Wallis-formula integral over [0,π/2], giving 643π.
Concept and Intuition
sin4(2x)cos6(2x) is a periodic function. Rather than grinding through a power-reduction expansion over the full range [−2π,2π], it's far more efficient to (a) substitute to a clean variable, (b) exploit periodicity to shrink the domain to one period, and (c) use the standard Wallis reduction formula for ∫0π/2sinmcosn.
Step-by-Step Solution
- Let u=2x⇒du=2dx. As x runs from −2π to 2π, u runs from −4π to 4π.
I=∫−2π2πsin4(2x)cos6(2x)dx=21∫−4π4πsin4ucos6udu.
- sin4ucos6u is unchanged under u→u+π (since sin(u+π)=−sinu, cos(u+π)=−cosu, and both powers are even), so it has period π.
- The interval [−4π,4π] has length 8π=8×π, i.e. exactly 8 full periods, so
∫−4π4πsin4ucos6udu=8∫0πsin4ucos6udu.
- On [0,π], the function is symmetric about u=π/2 (since sin(π−u)=sinu and cos(π−u)=−cosu, and cos6 is even in sign), so ∫0π=2∫0π/2.
- By the Wallis formula (both exponents even): …
- AP EAPCET 2023Set eng-2023-05-18-AN1 markMCQQ.∫−4π4πtan9xsin6xcos3xdx= (A) 16×2π (B) 8×32 (C) 16×1714×1512×…×32 (D) 0
›Reveal solutionSolution
Odd × even × even = odd, and the integral of any odd function over a symmetric interval is zero — no actual antiderivative work is needed.
Concept and Intuition
Before grinding through a nasty trig integral, always check parity. tan(−x)=−tanx (odd), and raising an odd function to an odd power (9) keeps it odd. sin(−x)=−sinx raised to an even power (6) becomes even, and cos(−x)=cosx raised to any power stays even. Odd times even times even is odd, and an odd function's graph is antisymmetric about the origin, so equal positive and negative area cancels exactly over any interval symmetric about 0.
Step-by-Step Solution
- Let g(x)=tan9xsin6xcos3x.
- g(−x)=tan9(−x)sin6(−x)cos3(−x)=(−tanx)9(sinx)6(cosx)3=−tan9xsin6xcos3x=−g(x).
- So g is an odd function. …
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