Q. Using the substitution x=sin2θ, or otherwise, evaluate ∫0211−xxdx.
Make Substitution: Let's make the substitution x=sin2(θ). Then, dx will be the derivative of sin2(θ) with respect to θ, which is 2sin(θ)cos(θ)d(θ) or sin(2θ)d(θ).
Change Limits: We need to change the limits of integration to match our substitution. When x=0, sin2(θ)=0, so θ=0. When x=21, sin2(θ)=21, so θ=4π. Therefore, our new limits of integration are from 0 to 4π.
Substitute and Simplify: Substitute x with sin2(θ) and dx with sin(2θ)d(θ) in the integral. The integral becomes:∫0π/41−sin2(θ)sin2(θ)⋅sin(2θ)d(θ)
Use Trigonometric Identity: Simplify the integral. Since 1−sin2(θ) is cos2(θ), the integral becomes:∫0π/4cos2(θ)sin2(θ)⋅sin(2θ)d(θ)This simplifies to:∫0π/4tan(θ)⋅sin(2θ)d(θ)
Cancel Terms: We can use a trigonometric identity to simplify sin(2θ) to 2sin(θ)cos(θ). The integral now becomes:∫0π/4tan(θ)⋅2sin(θ)cos(θ)d(θ)
Apply Double-Angle Identity: Since tan(θ) is sin(θ)/cos(θ), we can cancel out the cos(θ) in the denominator with one of the cos(θ) in the numerator. The integral simplifies to:∫0π/42sin2(θ)d(θ)
Split Integral: Use the double-angle identity for sine, which is sin2(θ)=21−cos(2θ). The integral becomes:∫04π2⋅21−cos(2θ)d(θ)This simplifies to:∫04π(1−cos(2θ))d(θ)
Integrate First Part: Split the integral into two parts and integrate each part separately: ∫0π/41dθ−∫0π/4cos(2θ)dθ
Integrate Second Part: Integrate the first part, which is the integral of 1 with respect to θ from 0 to π/4. This gives us:θ_{0}^{π/4} = π/4−0=π/4
Combine Results: Integrate the second part, which is the integral of cos(2θ) with respect to θ from 0 to π/4. This gives us:[21sin(2θ)](0)(π/4)=21sin(2π)−21sin(0)=21−0=21
Combine Results: Integrate the second part, which is the integral of cos(2θ) with respect to θ from 0 to π/4. This gives us:[21sin(2θ)](0)(π/4)=21sin(2π)−21sin(0)=21−0=21Combine the results of the two integrals to get the final answer. Subtract the result of the second integral from the first to get:π/4−21
Combine Results: Integrate the second part, which is the integral of cos(2θ) with respect to θ from 0 to π/4. This gives us:[21sin(2θ)](0)(π/4)=21sin(2π)−21sin(0)=21−0=21Combine the results of the two integrals to get the final answer. Subtract the result of the second integral from the first to get:π/4−21Simplify the final answer. Since 21 is the same as 2/4, we can write:π/4−2/4=(π−2)/4
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