Area 1

Calculus Level 2

If the area of region bounded by y 2 y^{2} =4-x and y 2 y^{2} =x is in form a b \frac{a}{b} c \sqrt{c} .Find a+b+c.(Where a and b are coprime positive integers).

21 25 18 36

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1 solution

Justin Tuazon
Oct 18, 2014

y 2 = 4 x y 2 = x x = 4 y 2 x = y 2 4 y 2 = y 2 y = ± 2 T h e r e f o r e , t h e c o m m o n p o i n t s a r e ± 2 T o f i n d t h e a r e a , 2 2 ( 4 y 2 y 2 ) d y = 2 0 2 ( 4 2 y 2 ) d y 2 [ 4 y 2 y 3 3 ] 0 2 = 2 ( 4 2 4 2 3 ) T h e a r e a i s 16 2 3 S o a = 16 , b = 3 , a n d c = 2 , T h e r e f o r e , a + b + c = 16 + 3 + 2 = 21 { y }^{ 2 }=4-x\\ { y }^{ 2 }=x\\ \Downarrow \\ x=4-{ y }^{ 2 }\\ x={ y }^{ 2 }\\ \Downarrow \\ 4-{ y }^{ 2 }={ y }^{ 2 }\\ y=\pm \sqrt { 2 } \\ \\ Therefore,\quad the\quad common\quad points\quad are\quad \pm \sqrt { 2 } \\ \\ To\quad find\quad the\quad area,\\ \\ \int _{ -\sqrt { 2 } }^{ \sqrt { 2 } }{ (4-{ y }^{ 2 }-{ y }^{ 2 })dy } =2\int _{ 0 }^{ \sqrt { 2 } }{ (4-2{ y }^{ 2 })dy } \\ \Downarrow \\ 2{ \left[ 4y-\frac { 2{ y }^{ 3 } }{ 3 } \right] }_{ 0 }^{ \sqrt { 2 } }=2(4\sqrt { 2 } -\frac { 4\sqrt { 2 } }{ 3 } )\\ \Downarrow \\ \quad The\quad area\quad is\frac { 16\sqrt { 2 } }{ 3 } \\ So\quad a=16,\quad b=3,\quad and\quad c=2,\\ \\ Therefore,\quad \boxed { a+b+c=16+3+2=21 }

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