MathematicsIntegration by SubstitutionJEE Advanced 1987Moderate
Visualized Solution (English)
Initial Integral and Double Angle Formula
- Given integral: I=∫sinx(cos2x)21dx
- Using the double angle identity: cos2x=cos2x−sin2x
- Substituting this into the integral: I=∫sinxcos2x−sin2xdx
Simplifying the Root and Transforming to Cotangent
- Bring sinx inside the square root: I=∫sin2xcos2x−sin2xdx
- Simplify the fraction inside the root: I=∫sin2xcos2x−sin2xsin2xdx
- Resulting form: I=∫cot2x−1dx
Choosing a Strategic Substitution
- To eliminate the square root, use the identity: sec2θ−1=tan2θ
- Let: cotx=secθ
- Then: cot2x−1=sec2θ−1=tanθ
Finding the Differential dx
- Differentiate cotx=secθ: −csc2xdx=secθtanθdθ
- Isolate dx: dx=−csc2xsecθtanθdθ
- Substitute csc2x=1+cot2x=1+sec2θ:
- Final differential: dx=−1+sec2θsecθtanθdθ
Substituting into the Integral
- Substitute back: I=∫(tanθ)(−1+sec2θsecθtanθ)dθ
- Combine terms: I=−∫1+sec2θsecθtan2θdθ
Simplifying the Integrand
- Convert to sine and cosine: I=−∫1+cos2θ1cosθ1⋅cos2θsin2θdθ
- Simplify the denominator: 1+cos2θ1=cos2θcos2θ+1
- Resulting expression: I=−∫cosθ(1+cos2θ)sin2θdθ
Preparing for Substitution u=sinθ
- Use cos2θ=1−sin2θ: I=−∫cosθ(1+1−sin2θ)sin2θdθ
- Simplify: I=−∫cosθ(2−sin2θ)sin2θdθ
- Multiply numerator and denominator by cosθ: I=−∫cos2θ(2−sin2θ)sin2θcosθdθ
- Substitute cos2θ=1−sin2θ again: I=−∫(1−sin2θ)(2−sin2θ)sin2θcosθdθ
Substitution and Partial Fractions
- Let u=sinθ, then du=cosθdθ
- Integral becomes: I=−∫(1−u2)(2−u2)u2du
- Using partial fractions: (1−u2)(2−u2)u2=1−u21−2−u22
- So: I=∫(2−u22−1−u21)du
Integrating the Terms
- Integrate ∫2−u22du=21log2−u2+u
- Integrate ∫1−u21du=21log1−u1+u=log∣secθ+tanθ∣
- Combined result: I=21log2−sinθ2+sinθ−log∣secθ+tanθ∣+C
Final Result in Terms of x
- Substitute sinθ=1−cos2θ=1−tan2x
- Substitute secθ=cotx and tanθ=cot2x−1
- Final Answer: I=21log2−1−tan2x2+1−tan2x−log(cotx+cot2x−1)+C
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