Inspired by Carl Lian

Calculus Level pending

Compute

0 π 3 sin θ + 4 cos θ + 4 25 cos θ + 7 d θ \int_0^\pi \frac{-3\sin \theta + 4 \cos \theta + 4}{25 \cos \theta + 7} d \theta

to the nearest thousandths.


The answer is 0.434.

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1 solution

Alan Yan
Feb 26, 2016

We let the quantity be \Re . We seek to make the expression homogeneous. Let θ = 2 x \theta = 2x . We have

= 2 0 π 2 3 sin 2 x + 4 cos 2 x + 4 25 cos 2 x + 7 d x = 2 0 π 2 6 sin x cos x + 4 ( 2 cos 2 x 1 ) 4 25 ( cos 2 x sin 2 x ) + 7 ( cos 2 x + sin 2 x ) d x = 2 0 π 2 6 sin x cos x + 8 cos 2 x 32 cos 2 x 18 sin 2 d x = 2 0 π 2 3 sin x cos x + 4 cos 2 x 16 cos 2 x 9 sin 2 d x = 2 0 π 2 cos x ( 4 cos x 3 sin x ) ( 4 cos x + 3 sin x ) ( 4 cos x 3 sin x ) d x = 2 0 π 2 cos x 4 cos x + 3 sin x d x = 2 0 π 2 4 25 ( 4 cos x + 3 sin x ) + 3 25 ( 4 cos x + 3 sin x ) ( 4 cos x + 3 sin x ) d x = 2 0 π 2 4 25 + 3 25 ( 4 cos x + 3 sin x ) ( 4 cos x + 3 sin x ) d x = 2 ( 4 x 25 + 3 25 ln ( 4 cos x + 3 sin x ) 0 π 2 ) 4.34 \begin{aligned} \Re & = 2 \int_0^\frac{\pi}{2} \frac{-3 \sin 2x + 4 \cos 2x + 4}{25 \cos 2x + 7} dx \\ & = 2 \int_0^\frac{\pi}{2} \frac{-6\sin x \cos x + 4(2\cos^2 x - 1) - 4}{25(\cos^2x - \sin^2 x) + 7(\cos^2 x + \sin^2 x)} dx\\ & = 2\int_0^\frac{\pi}{2}\frac{-6\sin x\cos x +8\cos^2 x}{32\cos^2x - 18\sin^2} dx = 2\int_0^\frac{\pi}{2}\frac{-3\sin x\cos x+4\cos^2 x}{16\cos^2x - 9\sin^2} dx \\ & = 2\int_0^\frac{\pi}{2} \frac{\cos x (4 \cos x - 3 \sin x)}{(4 \cos x + 3 \sin x)(4 \cos x - 3 \sin x)} dx = 2 \int_0^\frac{\pi}{2} \frac{\cos x}{4\cos x + 3\sin x} dx \\ & = 2\int_0^\frac{\pi}{2} \frac{ \frac{4}{25} (4 \cos x + 3 \sin x) + \frac{3}{25} (4 \cos x + 3 \sin x)'}{(4 \cos x + 3 \sin x)} dx \\ & = 2 \int_0^{\frac{\pi}{2}} \frac{4}{25} + \frac{3}{25} \cdot \frac{(4 \cos x + 3 \sin x)'}{(4 \cos x + 3 \sin x)} dx \\ & = 2\left ( \frac{4x}{25} + \frac{3}{25} \ln (4\cos x + 3 \sin x) \mid^{\frac{\pi}{2}}_0 \right ) \approx 4.34 \end{aligned}

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