It's all Symmetry.

Level pending

Let f ( x ) = x f(x) = \sqrt{|x|} and g ( x ) = x 2 g(x) = -x^2 .

If f ( x ) f(x) and g ( x ) g(x) have common tangents at points A A and B B and C C and D D respectively, find the area of the region above to eight decimal places.


The answer is 0.09640452.

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1 solution

Rocco Dalto
Dec 31, 2019

Let f ( x ) = x f(x) = \sqrt{|x|} and g ( x ) = x 2 g(x) = -x^2 and A : ( x 0 , y 0 ) A: (x_{0},y_{0}) .

R ( 0 , 0 ) R y = x ( x 0 , y 0 ) = ( y 0 , x 0 ) = B R_{(0,0)} \circ R_{y = x}(x_{0},y_{0}) = (-y_{0},-x_{0}) = B

and R y a x i s ( y 0 , x 0 ) = ( y 0 , x 0 ) = D R_{y - axis}(-y_{0},-x_{0}) = (y_{0},-x_{0}) = D and R y a x i s ( x 0 , y 0 ) = ( x 0 , y 0 ) = C R_{y - axis}(x_{0},y_{0}) = (-x_{0},y_{0}) = C

m A B = 1 \implies m_{AB} = 1 and m C D = 1 m_{CD} = -1

m A B = 1 f ( x 0 ) = 1 2 x 0 = 1 x 0 = 1 4 y 0 = 1 2 y = x + 1 4 m_{AB} = 1 \implies f'(x_{0}) = \dfrac{1}{2\sqrt{x_{0}}} = 1 \implies x_{0} = \dfrac{1}{4} \implies y_{0} = \dfrac{1}{2} \implies y = x + \dfrac{1}{4}

and m C D = 1 y = x + 1 4 m_{CD} = -1 \implies y = -x + \dfrac{1}{4}

To find the x x coordinate of the find point of intersection E E of y = x + 1 4 y = -x + \dfrac{1}{4} and f ( x ) = x f(x) = \sqrt{x}

let x + 1 4 = x 1 4 x = 4 x 1 8 x + 16 x 2 = 16 x 16 x 2 24 x + 1 = 0 x = 3 ± 2 2 4 -x + \dfrac{1}{4} = \sqrt{x} \implies 1 - 4x = 4\sqrt{x} \implies 1 - 8x + 16x^2 = 16x \implies 16x^2 - 24x + 1 = 0 \implies x = \dfrac{3 \pm 2\sqrt{2}}{4}

We drop x = 3 + 2 2 4 x = \dfrac{3 + 2\sqrt{2}}{4} which doesn't satisfy the initial equation and choose x = 3 2 2 4 x = \dfrac{3 - 2\sqrt{2}}{4} which satisfies the initial equation.

For A R 1 A_{R_{1}} we have:

A R 1 = 0 1 4 x + 1 4 x 1 2 d x = A_{R_{1}} = \displaystyle\int_{0}^{\frac{1}{4}} x + \dfrac{1}{4} - x^{\frac{1}{2}} \:\ dx = 1 2 x 2 + 1 4 x 2 3 x 3 2 0 1 4 = \dfrac{1}{2}x^2 + \dfrac{1}{4}x - \dfrac{2}{3}x^{\frac{3}{2}}|_{0}^{\frac{1}{4}} = 1 96 \boxed{\dfrac{1}{96}}

For A R 2 A_{R_{2}} we have:

A R 2 = 0 3 2 2 4 x 1 2 + x 2 d x = A_{R_{2}} = \displaystyle\int_{0}^{\frac{3 - 2\sqrt{2}}{4}} x^{\frac{1}{2}} + x^{2} \:\ dx = 2 3 x 3 2 + 1 3 x 3 0 3 2 2 4 = \dfrac{2}{3}x^{\frac{3}{2}} + \dfrac{1}{3}x^{3}|_{0}^{\frac{3 - 2\sqrt{2}}{4}} =

2 3 ( 3 2 2 4 ) 3 2 + 1 3 ( 3 2 2 4 ) 3 \boxed{\dfrac{2}{3}(\dfrac{3 - 2\sqrt{2}}{4})^{\frac{3}{2}} + \dfrac{1}{3}(\dfrac{3 - 2\sqrt{2}}{4})^{3}} .

For A R 3 A_{R_{3}} we have:

A R 3 = 3 2 2 4 1 2 ( x + 1 4 + x 2 ) d x = A_{R_{3}} = \displaystyle\int_{\frac{3 - 2\sqrt{2}}{4}}^{\frac{1}{2}} (-x + \dfrac{1}{4} + x^2) \:\ dx =

1 2 x 2 + 1 4 x + 1 3 x 3 3 2 2 4 1 2 = -\dfrac{1}{2}x^2 + \dfrac{1}{4}x + \dfrac{1}{3}x^3|_{\frac{3 - 2\sqrt{2}}{4}}^{\frac{1}{2}} =

1 24 ( 1 2 ( 3 2 2 4 ) 2 + 1 4 ( 3 2 2 4 ) + 1 3 ( 3 2 2 4 ) 3 ) \dfrac{1}{24} - (-\dfrac{1}{2}(\dfrac{3 - 2\sqrt{2}}{4})^2 + \dfrac{1}{4}(\dfrac{3 - 2\sqrt{2}}{4}) + \dfrac{1}{3}(\dfrac{3 - 2\sqrt{2}}{4})^{3})

A R 2 + A R 3 = ( 3 2 2 4 ) ( 8 3 2 2 + 3 6 2 24 ) + 1 24 A_{R_{2}} + A_{R_{3}} = (\dfrac{3 - 2\sqrt{2}}{4})(\dfrac{8\sqrt{3 - 2\sqrt{2}} + 3 - 6\sqrt{2}}{24}) + \dfrac{1}{24}

The Area A = A R 1 + A R 2 + A R 3 = A^{*} = A_{R_{1}} + A_{R_{2}} + A_{R_{3}} = 1 96 [ 5 + ( 3 2 2 ) ( 8 3 2 2 + 3 6 2 ) ] \dfrac{1}{96}[5 + (3 - 2\sqrt{2})(8\sqrt{3 - 2\sqrt{2}} + 3 - 6\sqrt{2})]

Using the symmetry about the y y axis the total area A = 2 A = A = 2A^{*} =

2 96 [ 5 + ( 3 2 2 ) ( 8 3 2 2 + 3 6 2 ) ] 0.09640452 \dfrac{2}{96}[5 + (3 - 2\sqrt{2})(8\sqrt{3 - 2\sqrt{2}} + 3 - 6\sqrt{2})] \approx \boxed{0.09640452}

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