Tranformation .

Calculus Level pending

What is the area bounded by the curve tan x + cot x tan x cot x \big| |\tan x+\cot x| - |\tan x - \cot x| \big| between the line x = 0 , x = π 2 x=0, x= \dfrac{\pi}{2} and the x x -axis?


The answer is 1.386.

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1 solution

The required area is given by:

A = 0 π 2 tan x + cot x tan x cot x d x = 0 π 4 tan x + cot x tan x cot x d x + π 4 π 2 tan x + cot x tan x cot x d x Note that tan x cot x for x [ 0 , π 4 ] = 0 π 4 tan x + cot x + tan x cot x d x + π 4 π 2 tan x + cot x tan x + cot x d x and tan x > cot x for x ( π 4 , π 2 ] = 2 0 π 4 tan x d x + 2 π 4 π 2 cot x d x = 2 ln ( cos x ) π 4 0 + 2 ln ( sin x ) π 4 π 2 = 2 ln 2 1.386 \begin{aligned} A & = \int_0^\frac \pi 2 \big| |\tan x + \cot x | - |\tan x - \cot x| \big|\ dx \\ & = \int_0^\frac \pi 4 \big| |\tan x + \cot x | - |\tan x - \cot x| \big|\ dx + \int_\frac \pi 4^\frac \pi 2 \big| |\tan x + \cot x | - |\tan x - \cot x| \big|\ dx & \small \color{#3D99F6} \text{Note that } \tan x \le \cot x \text{ for }x \in \left[0, \frac \pi 4\right] \\ & = \int_0^\frac \pi 4 |\tan x + \cot x + \tan x - \cot x |\ dx + \int_\frac \pi 4^\frac \pi 2 | \tan x + \cot x - \tan x + \cot x |\ dx & \small \color{#3D99F6} \text{and } \tan x > \cot x \text{ for }x \in \left(\frac \pi 4, \frac \pi 2 \right] \\ & = 2 \int_0^\frac \pi 4 \tan x \ dx + 2 \int_\frac \pi 4^\frac \pi 2 \cot x \ dx \\ & = 2\ln(\cos x) \bigg|_\frac \pi 4^0 + 2 \ln (\sin x) \bigg|_\frac \pi 4^\frac \pi 2 \\ & = 2 \ln 2 \approx \boxed{1.386} \end{aligned}

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