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Exercise 7.1 · Q11

Q.Integrate the following function: ∫x3+5x2−4x2dx\int \frac{x^3 + 5x^2 - 4}{x^2} dx

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The key idea is to split the rational function into simpler terms by dividing each term of the numerator by x2x^2, then integrate term-by-term using the power rule. The result is x22+5x+4x+C\frac{x^2}{2} + 5x + \frac{4}{x} + C.

When you see an integral like ∫x3+5x2−4x2dx\int \frac{x^3 + 5x^2 - 4}{x^2} dx, the natural instinct might be to look for a complicated substitution or partial fractions. But here, the denominator is just a single monomial — x2x^2. That means we can avoid long division entirely and simply split the fraction into separate terms.

Why does this work? Because a+b+cd=ad+bd+cd\frac{a+b+c}{d} = \frac{a}{d} + \frac{b}{d} + \frac{c}{d}, as long as d≠0d \neq 0. So we can rewrite the integrand as three simpler fractions, each of which is easy to integrate using the power rule.

Let’s do it step by step.

  1. Split the fraction

x3+5x2−4x2=x3x2+5x2x2−4x2\frac{x^3 + 5x^2 - 4}{x^2} = \frac{x^3}{x^2} + \frac{5x^2}{x^2} - \frac{4}{x^2}

  1. Simplify each term

x3x2=x3−2=x1=x\frac{x^3}{x^2} = x^{3-2} = x^1 = x

5x2x2=5x2−2=5x0=5\frac{5x^2}{x^2} = 5x^{2-2} = 5x^0 = 5

4x2=4x−2\frac{4}{x^2} = 4x^{-2}

So the integrand becomes:

x+5−4x−2x + 5 - 4x^{-2}

  1. Integrate term by term Using ∫xndx=xn+1n+1+C\int x^n dx = \frac{x^{n+1}}{n+1} + C for n≠−1n \neq -1:

∫x dx=x22\int x \, dx = \frac{x^2}{2}

∫5 dx=5x\int 5 \, dx = 5x

∫−4x−2 dx=−4⋅x−1−1=4x−1=4x\int -4x^{-2} \, dx = -4 \cdot \frac{x^{-1}}{-1} = 4x^{-1} = \frac{4}{x} …

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