Without calculator, is it possible to do?

Calculus Level 3

0 π 2 7 sin θ 4 cos θ + 3 2 sin θ + cos θ + 2 d θ = ? \large \int _{ 0 }^{ \frac { \pi }{ 2 } }{ \frac { 7\sin { \theta } -4\cos { \theta } +3 }{ 2\sin { \theta } +\cos { \theta } +2 } d\theta } = ?

Enter your answer with 3 significant figures.

Note: Try to not use calculator until you get the exact value of the integral.


The answer is 1.87.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Chew-Seong Cheong
Nov 29, 2017

I = 0 π 2 7 sin θ 4 cos θ + 3 2 sin θ + cos θ + 2 d θ Let t = tan θ 2 d t = sec 2 θ 2 d θ = 0 1 2 ( 7 ( 2 t ) 4 ( 1 t 2 ) + 3 ( 1 + t 2 ) ) ( 2 ( 2 t ) + 1 t 2 + 2 ( 1 + t 2 ) ) ( 1 + t 2 ) d t sin θ = 2 t 1 + t 2 , cos θ = 1 t 2 1 + t 2 = 0 1 14 t 2 + 28 t 2 ( t 2 + 4 t + 3 ) ( t 2 + 1 ) d t = 0 1 14 t 2 + 28 t 2 ( t + 1 ) ( t + 3 ) ( t 2 + 1 ) d t By partial fraction decomposition = 0 1 ( 6 t + 4 t 2 + 1 4 t + 1 2 t + 3 ) d t = 3 ln ( t 2 + 1 ) + 4 tan 1 t 4 ln ( t + 1 ) 2 ln ( t + 3 ) 0 1 = 3 ln 2 + π 4 ln 2 4 ln 2 3 ln 1 0 + 4 ln 1 + 2 ln 3 = π 5 ln 2 + 2 ln 3 1.87 \begin{aligned} I & = \int_0^\frac \pi 2 \frac {7\sin \theta - 4\cos \theta + 3}{2\sin \theta + \cos \theta +2} d\theta & \small \color{#3D99F6} \text{Let }t = \tan \frac \theta 2 \implies dt = \frac {\sec^2 \theta}2 d \theta \\ & = \int_0^1 \frac {2\left(7(2t)-4(1-t^2)+3(1+t^2)\right)}{\left(2(2t)+1-t^2+2(1+t^2)\right)(1+t^2)}dt & \small \color{#3D99F6} \implies \sin \theta = \frac {2t}{1+t^2}, \ \cos \theta = \frac {1-t^2}{1+t^2} \\ & = \int_0^1 \frac {14t^2+28t-2}{(t^2+4t+3)(t^2+1)}dt \\ & = \int_0^1 \frac {14t^2+28t-2}{(t+1)(t+3)(t^2+1)}dt & \small \color{#3D99F6} \text{By partial fraction decomposition} \\ & = \int_0^1 \left(\frac {6t+4}{t^2+1} - \frac 4{t+1} - \frac 2{t+3} \right) dt \\ & = 3\ln(t^2+1) + 4\tan^{-1} t - 4\ln(t+1) - 2\ln(t+3) \ \bigg|_0^1 \\ & = 3\ln 2 + \pi - 4\ln 2 - 4 \ln 2 - 3 \ln 1 - 0 + 4 \ln 1 + 2 \ln 3 \\ & = \pi - 5 \ln 2 + 2 \ln 3 \\ & \approx \boxed{1.87} \end{aligned}

Well solved....!!

Vishal Dbhat - 3 years, 6 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...