# Polar coor

## Homework Statement

Use polar coords to evaluate the double integral x3 + xy2dydx from y = -(9-x2)1/2 to (9-x2)1/2, and x = 0 to 3

## The Attempt at a Solution

So the region is a half circle of radius 3, centered @ the origin, with only the possitive x side, (right side of circle)

x = rcosQ, y = rsinQ

the integral is r3cos3Q + r3cosQsinQ drdQ

after integration with respect to r i get and pluging in the limits

84/4 cos^3 Q + 81/4 cosQsinQ dQ

i looked up the integrals of cos^3Q in the back of my book and it is quite complex. Have i made any mistakes thus far, and is there an easier way to evaluate it

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dx
Homework Helper
Gold Member
x³ + xy² = r² cosθ. Check your algebra.

Cyosis
Homework Helper
Also if you go from Cartesian to polar coordinates $dxdy \Rightarrow r dr d\theta$.
While not relevant to this problem $\cos^3x$ is easily integrated by writing it as $\cos x (1-\sin^2x)$ and substituting $u=\sin x$.

i think i messed up somehwere

$$\int r^3cosQdrdQ$$ = .25r^4cosQ from 0 to 3
=$$\int81/4 cosQ dQ$$ from 0 to pi = 81/4 sinQ from 0 to pi
sin(0) = 0, sin (pi) = 0
i got an answer of 0

dx
Homework Helper
Gold Member
Are you sure θ goes from 0 to π?

Cyosis
Homework Helper

You forgot that when you transform from Cartesian to polar coordinates you have to add the Jacobian r ($dx dy \Rightarrow r dr d\theta$). Secondly your limits for $\theta$ are off. You are integrating over the upper semi circle, but that is not the right region to integrate over. Draw the circle and add the Cartesian limits, over what section should you be integrating?

dx
Homework Helper
Gold Member

Q should go from $$-\frac{\pi}{2}$$ to $$\frac{\pi}{2}$$.

HallsofIvy
Homework Helper

You forgot that when you transform from Cartesian to polar coordinates you have to add the Jacobian r ($dx dy \Rightarrow r dr d\theta$). Secondly your limits for $\theta$ are off. You are integrating over the upper semi circle, but that is not the right region to integrate over. Draw the circle and add the Cartesian limits, over what section should you be integrating?
Actually, he did include the "r" but you are correct that his limits of integration on $\theta$ are wrong.

Cyosis
Homework Helper

You're right I completely missed the r.

uhhh huh, ok i didnt realize that mattered, i figured as long as it was 180 degrees it would be the same

but i think i did mess up the r

x(x2 + y2) = rcosQ* r2 = r3cosQ

then i add the jacobian r and my integral is for r4cosQdrdQ

the only thing that changes is the 81/4 becomes 243/5