Pair of Parabolas

Calculus Level 3

Let A be the area enclosed by the parabolas y = 4 x x 2 y=4x-{ x }^{ 2 } and y = x 2 x y=x^{ 2 }-x . Find 24A.


The answer is 125.

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2 solutions

The Area region is shown in the above graph.

The modified equations of the graphs are: y = 4 x x 2 i . e . , ( x 2 ) 2 = ( y 4 ) a n d y = x 2 x i . e . , ( x 1 2 ) 2 = y + 1 4 y = 4x - x^2 \ i.e., (x-2)^2 = -(y - 4) \ and \ y = x^2-x \ i.e., (x - \frac{1}{2})^2 = y + \frac{1}{4}

R e q u i r e d A r e a , A = A r ( O A E M ) A r ( D E M ) + A r ( O C D ) Required Area , A \ = \ Ar(OAEM) \ - \ Ar(DEM) \ + \ Ar(OCD) = 0 5 2 ( 4 x x 2 ) d x 1 5 2 ( x 2 x ) d x 0 1 ( x 2 x ) d x = \int\limits_0^\frac{5}{2} (4x - x^2) dx - \int\limits_1^\frac{5}{2} (x^2 - x) dx - \int\limits_0^1 (x^2 - x) dx [ 2 x 2 x 3 3 ] 0 5 2 [ x 3 3 x 2 2 ] 1 5 2 [ x 3 3 x 2 ] 0 1 \left[ { 2x^2 - \frac{x^3}{3} } \right] _{ 0 }^{ \frac{5}{2} } - \left[ {\frac{x^3}{3}-\frac{x^2}{2} } \right] _{ 1 }^{ \frac{5}{2} } - \left[ {\frac{x^3}{3}-\frac{x}{2} } \right] _{ 0 }^{ 1 } = [ 2 ( 25 4 ) ( 5 2 ) 3 3 ] [ ( 5 2 ) 3 1 3 3 ( 5 2 ) 2 1 2 2 ] [ 1 3 3 1 2 ] = \left[2(\frac{25}{4}) - \frac{(\frac{5}{2})^3}{3}\right] - \left[ \frac{(\frac{5}{2})^3 - 1^3}{3} - \frac{(\frac{5}{2})^2 - 1^2}{2}\right] - \left[\frac{1^3}{3} - \frac{1}{2}\right] = 300 125 125 + 8 + 75 12 8 + 12 24 = \frac{300 - 125 - 125 + 8 +75 -12 - 8 +12}{24} = 125 24 s q . u n i t s = \frac{125}{24} sq. units

24 A = 24 ( 125 24 ) = 125 s q . u n i t s \Rightarrow 24A = 24(\frac{125}{24}) = \boxed{125 \ sq. units}

Well explained... I guess I need not explain the solution now...

Zeeshan Ali - 5 years, 5 months ago
Eliott Collin
Dec 27, 2014

Graphically, we clearly see that A = ( 0 2.5 ( 4 x x 2 ) d x ) ( 1 2.5 ( x 2 x ) d x ) ( 0 1 ( x 2 x ) d x ) A=(\int_0^{2.5} (4x-x^2) dx)-(\int_1^{2.5} (x^2-x) dx)-(\int_0^1 (x^2-x) dx)

We then find A = 125 / 24 A=125/24 . Hence, 24 A = 125 / 24 24 = 125 24A=125/24*24 \boxed{=125}

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