Area between two parabolas - 2

Calculus Level 1

Two parabolas are given by y = 1 16 ( x 3 ) 2 + 5 y = \frac{1}{16} (x - 3)^2 + 5 , and y = 1 2 ( x 5 ) 2 + 1 y = \frac{1}{2} (x - 5)^2 + 1 . Find the area bounded by the two curves.


The answer is 17.88.

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1 solution

Let us first check where the two parabolas intersect. When they intersect, we have:

( x 3 ) 2 16 + 5 = ( x 5 ) 2 2 + 1 x 2 6 x + 89 16 = x 2 10 x + 27 2 x 2 6 x + 89 = 8 x 2 80 x + 216 7 x 2 74 x + 127 = 0 x = 37 ± 4 30 7 \begin{aligned} \frac {(x-3)^2}{16}+5 & = \frac {(x-5)^2}2+1 \\ \frac {x^2-6x+89}{16} & = \frac {x^2-10x+27}2 \\ x^2-6x+89 & = 8x^2-80x+216 \\ 7x^2 - 74x + 127 & = 0 \\ \implies x & = \frac {37 \pm 4\sqrt{30}}7 \end{aligned}

Then the area bounded by the two parabolas is

A = 37 4 30 7 37 + 4 30 7 ( ( x 3 ) 2 16 + 5 ( x 5 ) 2 2 1 ) d x = 1 16 37 + 4 30 7 37 4 30 7 ( 7 x 2 74 x + 127 ) d x = 1 16 [ 7 3 x 3 37 x 2 + 127 x ] 37 + 4 30 7 37 4 30 7 17.9 \begin{aligned} A & = \int_{\frac {37-4\sqrt{30}}7}^{\frac {37+4\sqrt{30}}7} \left(\frac {(x-3)^2}{16}+5 - \frac {(x-5)^2}2 - 1 \right) dx \\ & = \frac 1{16} \int_{\frac {37+4\sqrt{30}}7}^{\frac {37-4\sqrt{30}}7} \left(7x^2 - 74x + 127 \right) dx \\ & = \frac 1{16} \left[\frac 73 x^3 - 37x^2 + 127x \right]_{\frac {37+4\sqrt{30}}7}^{\frac {37-4\sqrt{30}}7} \\ & \approx \boxed{17.9} \end{aligned}

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