Calculate the area when $\displaystyle \frac{x^2}{4}+\frac{y^2}{2}=1$ is rotated around the y-axis.

This is the formula. $\displaystyle A_y =2\pi\int_a^b x \sqrt{1+\left(\frac{dx}{dy}\right)^2} \, dy$ But when I solve for y, I get a ±. Should I just take the +version?