Calculating area

Calculus Level 5

x = 17 cos 3 t , y = 5 sin 3 t \large x = 17 \cos^3 t, y = 5 \sin^3 t

The whole area bounded by curves given above is given by a π b \frac{a\pi}{b} where a , b a,b are co prime positive integers.

Find a + b a+b .


The answer is 263.

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1 solution

Given equations : x = 17 c o s 3 t {x=17cos^{3}t} and y = 5 s i n 3 t \large{y=5sin^{3}t} are parametric equations of the curve:

( x 2 a 2 ) 1 3 + ( y 2 b 2 ) 1 3 = 1 { \left( \frac { { x }^{ 2 } }{ { a }^{ 2 } } \right) }^{ \frac { 1 }{ 3 } }+{ \left( \frac { { y }^{ 2 } }{ { b }^{ 2 } } \right) }^{ \frac { 1 }{ 3 } }=1

A r e a = 4 ( a r e a i n 1 s t q u a d r a n t ) \therefore Area \ = \ 4 \ (area \ in \ 1st \ quadrant)

A r e a = 4 π 2 0 ( 5 s i n 3 t ) ( 3 × 17 c o s 2 t . s i n t ) d t \Rightarrow Area \ = \ 4\int _{ \frac { \pi }{ 2 } }^{ 0 }{ (5{ sin }^{ 3 }t)(-3\times 17{ cos }^{ 2 }t.sint) } dt

= 12 × 17 × 5 0 π 2 s i n 4 t . c o s 2 t d t = \ 12\times 17\times 5\int _{ 0 }^{ \frac { \pi }{ 2 } }{ { sin }^{ 4 }t.{ cos }^{ 2 }t } dt

On solving further we get:

= 1020. 3.1.1 6.4.2 . π 2 = 255 π 8 =\quad 1020.\frac { 3.1.1 }{ 6.4.2 } .\frac { \pi }{ 2 } =\quad \large{\boxed{\frac { 255\pi }{ 8 }}}

a + b = 255 + 6 = 263 \Rightarrow a\quad +\quad b\quad =\quad 255\quad +\quad 6\quad =\quad \boxed{263}

Moderator note:

Good work. Fun fact: this graph resembles an astroid .

nice solution @Harshvardhan Mehta

Tanishq Varshney - 6 years, 1 month ago

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thank you... ¨ \ddot \smile btw where did you get this question from??? or is it your original one??

Harshvardhan Mehta - 6 years, 1 month ago

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i found it in my friends test paper.

Tanishq Varshney - 6 years, 1 month ago

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