Q.Integrate the following function: sin3xcos3x
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Sine Double Angle Integration
Sine Double Angle Integration — From Intuition to Formula
Suppose you want the area under sin(2x) from x=0 to π/2. This graph oscillates twice as fast as a regular sine wave, completing a cycle in π units instead of 2π — the "double angle" inside compresses the wave horizontally.
The catch: you can't integrate sin(2x) the same way as sinx. Differentiating cos(2x) gives −2sin(2x) — not −sin(2x) — so the antiderivative needs a factor to compensate for that extra 2.
The Precise Statement
∫sin(ax)dx=−a1cos(ax)+C
For a=2:
∫sin(2x)dx=−21cos(2x)+C
This is the sine double angle integration formula — a direct application of the reverse chain rule.
Why It Works
Differentiate the right-hand side:
dxd[−21cos(2x)+C]=−21⋅(−sin(2x))⋅2=sin(2x)
The −21 cancels the −2 from the chain rule, leaving exactly sin(2x).
A common mistake is writing ∫sin(2x)dx=−cos(2x)+C. Differentiating −cos(2x) gives 2sin(2x), not sin(2x). Always check by differentiating your answer.
Definite Integrals
∫absin(2x)dx=[−21cos(2x)]ab=−21[cos(2b)−cos(2a)]
Example: ∫0π/2sin(2x)dx=−21[cos(π)−cos(0)]=−21[(−1)−1]=1.
The General Pattern
∫sin(kx)dx=−k1cos(kx)+C
The k in the denominator is the "compensation factor" for the chain rule, working for any constant k=0. …
Write everything with the double angle: sinxcosx=21sin2x.
sin3xcos3x=(sinxcosx)3=(21sin2x)3=81sin32x.
For ∫sin32xdx, write sin32x=(1−cos22x)sin2x and let u=cos2x, du=−2sin2xdx:
∫sin32xdx=−21∫(1−u2)du=−21(u−3u3)=−21cos2x+61cos32x.
Multiply by 81: …
Since sin3xcos3x=81sin32x, integrating gives −161cos2x+481cos32x+C.
Compress with the double angle
Both factors share the same power, so group them: sin3xcos3x=(sinxcosx)3. Using sinxcosx=21sin2x,
sin3xcos3x=(21sin2x)3=81sin32x,
so ∫sin3xcos3xdx=81∫sin32xdx.
Odd power of sine: save one factor
sin32x=sin22x⋅sin2x=(1−cos22x)sin2x. The spare sin2x is perfect for the substitution u=cos2x, since du=−2sin2xdx, i.e. sin2xdx=−21du:
∫sin32xdx=∫(1−u2)(−21du)=−21(u−3u3)+C1=−21cos2x+61cos32x+C1.
Restore the 81
∫sin3xcos3xdx=81(−21cos2x+61cos32x)+C=−161cos2x+481cos32x+C. …
Method: Equal odd powers of sin and cos — compress with the double angle
For sinmxcosnx where both powers are equal and odd, group them as (sinxcosx)m, use sinxcosx=21sin2x, then handle the resulting odd power of sin2x by the save-one-factor substitution.
Steps
Step 1: Group the equal powers.
sin3xcos3x=(sinxcosx)3
Step 2: Collapse with the double-angle identity.
sinxcosx=21sin2x ⇒ (sinxcosx)3=81sin32x
Step 3: Integrate the odd power of sin2x by substitution. …
Common Mistakes
Mistake 1: Trying the power rule on sin3xcos3x as if it were a simple power.
Why it's wrong: neither sinx nor cosx has its derivative sitting alone as a factor of the whole product, so no single-step substitution or power rule applies. Correct approach: compress via (sinxcosx)3=81sin32x, then substitute u=cos2x.
Mistake 2: Forgetting the −2 in du=−2sin2xdx when substituting. …
Showing the 12 most recent of 17 on this concept.
- CBSE 20241 markMCQQ.∫sin2xcos2xcos2xdx is equal to : (A) cotx+tanx+c (B) −cotx+tanx+c (C) cotx−tanx+c (D) −cotx−tanx+c
›Reveal solutionSolution
The integral simplifies by rewriting cos2x as cos2x−sin2x, splitting into two simple integrals, and integrating term by term. The result is −cotx−tanx+c, which matches option (D).
The key insight here is that the denominator sin2xcos2x is a product of squares, and the numerator cos2x is begging to be expressed in terms of sin2x and cos2x. Once you do that, the fraction splits naturally into two separate terms, each of which is a standard integral.
Let’s walk through it.
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Rewrite cos2x using a double-angle identity.
The most useful form here is cos2x=cos2x−sin2x. This directly matches the squares in the denominator.
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Split the integrand into two fractions.
sin2xcos2xcos2x=sin2xcos2xcos2x−sin2x=sin2xcos2xcos2x−sin2xcos2xsin2x
Cancel common factors:
=sin2x1−cos2x1
- Recognise the standard forms.
sin2x1=csc2x,cos2x1=sec2x
So the integral becomes:
∫(csc2x−sec2x)dx
- Integrate term by term. Recall:
∫csc2xdx=−cotx+c,∫sec2xdx=tanx+c
Therefore:
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- CBSE 2026Set ANNUAL1 markMCQQ.∫0π/21+cos2xdx=(a) 0(b) 1(c) 21(d) None of these
›Reveal solutionSolution
Use 1+cos2x=2cos2x to simplify the square root, then integrate cosx.
Since 1+cos2x=2cos2x, we have 1+cos2x=2∣cosx∣. On [0,π/2], cosx≥0, so this is 2cosx.
…
- CBSE 2025Set ANNUAL1 markMCQQ.Value of ∫cos2xdx is:(a) 41sin2x+c(b) 2x+41sin2x+c(c) sin2x+c(d) 2x−41cos2x+c
›Reveal solutionSolution
Use the identity cos2x=21+cos2x.
…
- CBSE 2025Set E1 markMCQQ.∫0π/2sinx⋅cosxdx=(a) 1(b) 21(c) −1(d) 41
›Reveal solutionSolution
With u=sinx, ∫0π/2sinxcosxdx=[2sin2x]0π/2=21.
Let u=sinx, so du=cosxdx. Then
…
- CBSE 2025Set ANNUAL1 markMCQQ.∫0π/4sin2xdx is equal to(a) 0(b) 1(c) 2(d) 21
›Reveal solutionSolution
Antiderivative of sin2x is −cos2x/2; evaluating from 0 to π/4 gives 1/2.
∫0π/4sin2xdx=[−2cos2x]0π/4
…
- CBSE 2024Set D1 markMCQQ.∫0π/2cos2xdx=(a) 0(b) 1(c) −1(d) 2
›Reveal solutionSolution
The antiderivative 2sin2x evaluates to 0 at both limits.
…
- CBSE 2024Set ANNUAL1 markMCQQ.∫sin2xdx=(a) 2x−4sin2x+c(b) 4sin2x−2x+c(c) 2sin2x+2x+c(d) none of these
›Reveal solutionSolution
Rewrite sin^2 x using the double-angle identity, then integrate term by term.
Using sin2x=21−cos2x: …
- CBSE 2023Set AX1 markMCQQ.The value of ∫cos2xdx will be(a) 2x+41sin2x+c(b) 4x−21sin2x+c(c) cos2x−sin2x+c(d) 2cosxsinx+2x+c
›Reveal solutionSolution
Reduce the square with the double-angle identity, then integrate term by term: 2x+41sin2x+c, option (a).
Concept. A squared trig function is integrated by lowering the power with a double-angle identity, turning cos2x into a simple linear combination of 1 and cos2x.
Solution. Using cos2x=21+cos2x, …
- CBSE 2023Set ANNUAL1 markMCQQ.The value of ∫cos2xdx is:(a) 2x+41sin2x+C(b) x2+41sin2x+C(c) 4x+21sinx+C(d) 2x2+21sin2x+C
›Reveal solutionSolution
Use the identity cos2x=21+cos2x to make the integral straightforward.
∫cos2xdx=∫21+cos2xdx=21∫dx+21∫cos2xdx
…
- CBSE 2023Set ANNUAL1 markQ.Evaluate : ∫0π/2cos2xdx OR Evaluate : ∫011+x2dx
›Reveal solutionSolution
Integrate cos2x to 21sin2x and evaluate the limits.
∫0π/2cos2xdx=[2sin2x]0π/2=2sinπ−2sin0=20−20=0.
…
- CBSE 2022Set ANNUAL1 markMCQQ.∫π/2π/2sin2xdx=(a) 0(b) 1(c) 5(d) 11
›Reveal solutionSolution
The limits of integration are identical, so the integral is 0.
The printed integral is ∫π/2π/2sin2xdx. By the property ∫aaf(x)dx=0, any definite integral whose lower and upper …
- CBSE 2021Set I1 markMCQQ.∫0π/2cosxdx=(a) 1(b) −1(c) 0(d) 2
›Reveal solutionSolution
Antiderivative of cosx is sinx; evaluate at the limits.
…
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