# Parametric representations and surface area

#### LoffieZA

##### New member
Hello, I am looking for someone to check my work for me. Please see the attached questions and my work. It deals with surface area and parametric representations.

Thank you

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#### HallsofIvy

##### Elite Member
Your solutions to problem 1 and 2.1 are correct. For 2.2 you have calculated "dS" using the length of the normal vector at the single point $$\displaystyle (4, -2, 0)$$ where u= 2 and $$\displaystyle v= \pi$$. You can't do that. It has to be at any u, v. So $$\displaystyle dS= \sqrt{ u^2+ 4u^2cos^2(v)+ 4u^2sin^2(v)}dudv=\sqrt{5u^2}dudv= u\sqrt{5}dudv$$. Yes, $$\displaystyle x(y^2+z^2)= 2u(u^2cos^2(v)+ u^2sin^2(v))= 2u(u^2)= 2u^3$$ (it seems more work that necessary to do it as $$\displaystyle xy^2+ xz^2$$). So the integral is $$\displaystyle 2\sqrt{5}\int_0^{2\pi}\int_0^3 u^4 dudv$$.

For problem 3 you have one notational errors, perhaps a "typo". You have "$$\displaystyle dS= \sqrt{14}$$" when it should be, of course, $$\displaystyle dS= \sqrt{14}dxdy$$. But you do have the correct integral.

Problem 4 looks good.

• topsquark

Thank you!