Consider an ellipse of equation + = that contains the circle
The question is: Wich values of a and b minimize the area of the ellipse?
like HallsofIvy said, "you want your ellipse to just contain that circle which means it must be tangent". In other words, they'll have the same . This suggests you to take the derivative.
Futhermore, you also need to realize that at that point y value shall be the same.
So, solve in circlue equation in terms of .
Then, plug that value in of the ellispe equation.
Now write the ellipse equation for y in terms of a and b (remember you already eliminated x by substitution).
Next, use implicit differentiation on both equations.
edit: that's an excellent idea TheAbstractionist gave you. Should be lot faster than what i said