Transformation Complication

Calculus Level 5

If the area of the blue region bounded by the graph y = x 2 y = x^2 and its image after the transformation matrix presented by ( 2 1 3 2 ) \begin{pmatrix} 2 & 1 \\ 3 & 2 \end{pmatrix} can be expressed as A B \dfrac{A}{B} , where A , B A,B are positive integers and gcd ( A , B ) = 1 \gcd (A,B) = 1 , input A + B A + B as your answer.


The answer is 26.

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1 solution

First, I think the picture isn't quite right in that it was horizontally reflected.

Ok, let's get to the solution.

Step 1: Find the equation of the transformed graph x = 2 x + y x'=2x+y y = 3 x + 2 y y'=3x+2y

The original equation: y = x 2 y=x^2

The transformed graph: 3 x + 2 y = ( 2 x + y ) 2 3x+2y=(2x+y)^2

Step 2: Solve for points of intersection between these two graphs via simultaneous equations y = x 2 y=x^2 3 x + 2 y = ( 2 x + y ) 2 3x+2y=(2x+y)^2 Which is relatively straightforward, giving us 4 coordinates: ( 0 , 0 ) , ( 3 , 9 ) , ( 1 + ( 5 ) 2 , 3 ( 5 ) 2 ) , ( 1 ( 5 ) 2 , 3 + ( 5 ) 2 ) (0,0),(-3,9),(\frac{-1+\sqrt(5)}{2},\frac{3-\sqrt(5)}{2}),(\frac{-1-\sqrt(5)}{2},\frac{3+\sqrt(5)}{2})

Step 3: Make y the subject in the equation of the transformed graph y = 2 x ± 1 x + 1 y=-2x\pm \sqrt{1-x}+1

Step 4: Find multiples areas, which will be added/subtracted with each other to find the final area

Let the final area be X

Let A,B,C,D be these areas A = 3 1 5 2 x 2 d x A=\int_{-3}^{\frac{-1-\sqrt{5}}{2}} \!x^2\, \mathrm{d}x B = 1 5 2 0 2 x 1 x + 1 d x B=\int_{\frac{-1-\sqrt{5}}{2}}^{0} \!-2x-\sqrt{1-x}+1\, \mathrm{d}x --> chose minus because looking at the lower curve C = 0 1 + 5 2 x 2 d x C=\int_{0}^{\frac{-1+\sqrt{5}}{2}} \!x^2\, \mathrm{d}x D = 3 1 + 5 2 2 x + 1 x + 1 d x D=\int_{-3}^{\frac{-1+\sqrt{5}}{2}} \!-2x+\sqrt{1-x}+1\, \mathrm{d}x --> chose plus because looking at the upper curve

Evaluating all 4 of these, we get A = 25 5 3 , B = 4 + 5 3 , C = 2 + 5 3 , D = 50 + 5 3 A=\frac{25-\sqrt{5}}{3},B=\frac{4+\sqrt{5}}{3},C=\frac{-2+\sqrt{5}}{3},D=\frac{50+\sqrt{5}}{3}

Step 5: Calculate the final area X = D A B C = 23 3 X=D-A-B-C=\frac{23}{3}

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