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Calculus Level 5

Find the area of region bounded by the curve 2 x y + 2 x 1 1 2^{|x|}|y|+2^{|x|-1}\leq 1 ,within the square formed by the line x 1 2 , y 1 2 |x|\leq\frac{1}{2},|y|\leq\frac{1}{2}


The answer is 0.6902.

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1 solution

Chew-Seong Cheong
Mar 19, 2016

The equation 2 x y + 2 x 1 = 1 2^{|x|}|y| + 2^{|x|-1} = 1 is an even on both x x and y y . There equation has 4 4 curves symmetrical on x- and y-axes. The area bounded by the inequality 2 x y + 2 x 1 1 2^{|x|}|y| + 2^{|x|-1} \le 1 and x 1 2 |x| \le \frac{1}{2} or 1 2 x 1 2 -\frac{1}{2} \le x \le \frac{1}{2} is 4 × 4 \times that of under the curve 2 x y + 2 x 1 = 1 2^x y + 2^{x-1} = 1 for 0 x 1 2 0 \le x \le \frac{1}{2} . We note that 2 x y + 2 x 1 = 1 2^x y + 2^{x-1} = 1 , y = 1 2 x 1 2 \Rightarrow y = \dfrac{1}{2^x} - \frac{1}{2} and that y 1 2 y \le \frac{1}{2} for all x x . There the area A A bounded by the inequality 2 x y + 2 x 1 1 2^{|x|}|y| + 2^{|x|-1} \le 1 , x 1 2 |x| \le \frac{1}{2} and y 1 2 \color{#3D99F6}{|y| \le \frac{1}{2}} is same as that bounded by the inequality and x 1 2 |x| \le \frac{1}{2} . Therefore, we have:

A = 4 0 1 2 y d x = 4 0 1 2 ( 2 x 1 2 ) d x = 4 [ 2 x ln 2 x 2 ] 0 1 2 = 4 ( 1 1 2 ln 2 1 4 ) 0.6902 \begin{aligned} A & = 4 \int_0^{\frac{1}{2}} y \space dx \\ & = 4 \int_0^{\frac{1}{2}} \left(2^{-x} -\frac{1}{2}\right) dx \\ & = 4 \left[-\frac{2^{-x}}{\ln 2} - \frac{x}{2}\right]_0^{\frac{1}{2}} \\ & = 4 \left( \frac{1-\frac{1}{\sqrt{2}}}{\ln 2} - \frac{1}{4} \right) \approx \boxed{0.6902} \end{aligned}

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