Area of Row

Level 2

The letter R R is composed of the following curves:

The region bounded by x = ( y 6 ) ( y 3 ) x = -(y - 6)(y - 3) and the positive y y axis from y = 3 y = 3 to y = 6 y = 6 .

The region bounded by y = 3 2 x + 3 y = -\dfrac{3}{2}x + 3 , 0 y 3 0 \leq y \leq 3 and 0 x 2 0 \leq x \leq 2 .

The letter O O is the region of the ellipse ( x 4 ) 2 + ( y 3 ) 2 9 = 1 (x - 4)^2 + \dfrac{(y - 3)^2}{9} = 1 .

For the letter W W with the line y = 6 y = 6 , reflect the lines x = 42 y 6 x = \dfrac{42-y}{6} from y = 0 y = 0 to y = 6 y = 6 and x = y + 21 3 x = \dfrac{y + 21}{3} from y = 0 y = 0 to y = 3 y = 3 about the line x = 8 x = 8 forming the desired bounded region..

Find the Area of R O W ROW . That is, find the sum of the areas to six decimal places.


The answer is 43.924777.

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1 solution

Rocco Dalto
Mar 19, 2018

For the letter R R above:

A R = 3 6 y 2 9 y + 18 d y + 3 = ( y 3 3 9 y 2 2 + 18 y ) 3 6 + 3 = 3 + 9 2 = 15 2 A_{R} = -\int_{3}^{6} y^2 - 9y + 18 dy + 3 = -(\dfrac{y^3}{3} -\dfrac{9y^2}{2} + 18y)|_{3}^{6} + 3 = 3 + \dfrac{9}{2} = \boxed{\dfrac{15}{2}} .

For the letter O O using the ellipse above:

A e = 6 3 5 1 ( x 4 ) 2 + 1 d x A_{e} = 6\int_{3}^{5} \sqrt{1 - (x - 4)^2} + 1 dx

Let x 4 sin ( θ ) d x = cos ( θ ) d θ A e = 6 ( 1 2 ( θ + 1 2 sin ( 2 θ ) ) π 2 π 2 + x 3 5 ) = 3 ( π + 4 ) x - 4 - \sin(\theta) \implies dx = \cos(\theta) d\theta \implies A_{e} = 6(\dfrac{1}{2}(\theta + \dfrac{1}{2}\sin(2\theta))|_{\dfrac{-\pi}{2}}^{\dfrac{\pi}{2}} + x|_{3}^{5}) = \boxed{3(\pi + 4)}

For the letter W W with the line y = 6 y = 6 :

Using the symmetry about the line x = 8 x = 8 :

On the left side of the line x = 8 x = 8 we have a right triangle and a trapezoid A W = 2 ( 3 + 9 2 ) = 15 \implies A_{W} = 2(3 + \dfrac{9}{2}) = \boxed{15}

\therefore The sum of the areas A R O W = 6 π + 69 2 43.924777 A_{ROW} = \dfrac{6\pi + 69}{2} \approx \boxed{43.924777} .

Note: Using the definite integral for the letter W W with the line y = 6 y = 6 we obtain:

Again, using the symmetry about the line x = 8 x = 8 \implies A W = 2 ( 0 6 y + 21 3 ( 42 y 6 ) d y 3 6 8 ( y + 21 3 ) d y ) = A_{W} = 2(\int_{0}^{6} \dfrac{y + 21}{3} - (\dfrac{42 - y}{6}) dy - \int_{3}^{6} 8 - (\dfrac{y + 21}{3}) dy) =

2 ( 1 4 y 2 0 6 1 3 ( 3 y y 2 2 ) 3 6 = 2 ( 9 3 2 ) = 15 . 2(\dfrac{1}{4}y^2|_{0}^{6} - \dfrac{1}{3}(3y-\dfrac{y^2}{2})|_{3}^{6} = 2(9 - \dfrac{3}{2}) = \boxed{15}.

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