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Calculus Level 3

0 1 sin ( x ) cos ( x ) tan ( x ) d x \large \int_{0}^{1} \dfrac{\sin(x)\cos(x)}{\tan(x)} \ dx

Find the value of the above definite integral to two decimal places.


The answer is 0.73.

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2 solutions

Ashish Menon
Jun 6, 2016

sin ( x ) cos ( x ) tan ( x ) = sin ( x ) cos ( x ) sin ( x ) cos ( x ) = cos 2 ( x ) \dfrac{\sin(x)\cos(x)}{\tan(x)} = \dfrac{\sin(x)\cos(x)}{\frac{\sin(x)}{\cos(x)}} = {\cos}^2(x) .
Now, cos ( 2 x ) = cos 2 ( x ) sin 2 ( x ) cos ( 2 x ) = 2 cos 2 ( x ) cos 2 ( x ) sin 2 ( x ) cos ( 2 x ) = 2 cos ( x ) ( cos 2 ( x ) + sin 2 ( x ) ) cos ( 2 x ) = 2 cos 2 ( x ) 1 cos 2 ( x ) = 1 + cos ( 2 x ) 2 \cos(2x) = {\cos}^2(x) - {\sin}^2(x)\\ \cos(2x) = {2\cos}^2(x) - {\cos}^2(x) - {\sin}^2(x)\\ \cos(2x) = {2\cos}^(x) - \left({\cos}^2(x) + {\sin}^2(x)\right)\\ \cos(2x) = {2\cos}^2(x) - 1\\ {\cos}^2(x) = \dfrac{1 + \cos(2x)}{2}

So, the integral reduces to 0 1 1 + cos ( 2 x ) 2 d x \int_{0}^{1} \dfrac{1 + \cos(2x)}{2} \ dx = 1 2 0 1 1 + cos ( 2 x ) d x = 1 2 ( 0 1 1 d x + 0 1 cos ( 2 x ) d x ) = 1 2 ( 1 + sin ( 2 ) 2 ) = 2 + sin ( 2 ) 4 0.73 = \dfrac{1}{2} \int_{0}^{1} 1 + \cos(2x) \ dx\\ \\ = \dfrac{1}{2}\left(\int_{0}^{1} 1 \ dx + \int_{0}^{1} \cos(2x) \ dx\right)\\ \\ = \dfrac{1}{2}\left(1 + \dfrac{\sin(2)}{2}\right)\\ \\ = \dfrac{2 + \sin(2)}{4}\\ \\ \approx \color{#3D99F6}{\boxed{0.73}}

I = 0 1 sin x cos x tan x d x = 0 1 sin x cos x sin x cos x d x = 0 1 cos 2 x d x cos ( 2 x ) = 2 cos 2 x 1 = 0 1 1 + cos ( 2 x ) 2 d x = 1 2 ( x + sin ( 2 x ) 2 ) 0 1 = 1 2 ( 1 + sin 2 2 ) 0.73 \begin{aligned} I & = \int_0^1 \frac {\sin x \cos x} {\tan x} dx \\ & = \int_0^1 \frac {\sin x \cos x} {\dfrac {\sin x} {\cos x }} dx \\ & = \int_0^1 \color{#3D99F6}{\cos^2 x} \ dx \quad \quad \small \color{#3D99F6}{\cos (2x)=2\cos ^2 x-1} \\ & = \int_0^1 \color{#3D99F6} {\frac {1 +\cos (2x)} {2}} dx \\ & = \frac 12 \left(x+\frac {\sin(2x) } {2} \right) \bigg|_0^1 \\ & = \frac 12\left(1+\frac {\sin 2} {2} \right) \\& \approx \boxed {0.73}\end{aligned}

Integrate root(cos2t)/sint dt

Deepak Kumar - 5 years ago

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