4) Find the area of the surface obtained by rotating the curve from to about the -axis.
Any idea about the answer? I try to do it and my answer always turn out wrong. I don't know why
Almost but not quite.
Let u= 5x+ 25/4. Then du= 5dx so that dx= (1/5)du and the integral is $\displaystyle (1/5)\int u^{1/2}du$. You don't have the "1/5" and the integral of $\displaystyle u^{1/2}$ is [tex](2/3)u^{3/2}[/itex]. It looks like you differentiated rather that integrating.