Consider an isosceles triangle whose two equal sides are of length 5. What is the largest possible area for such a triangle?
Have you tried to draw a diagram? Just draw the triangle with the sides equal to 5, and treat the base as a variable x. Try to derive an expression for the area as a function of x. That's the best way to start on a problem like this.
Yes. The area formula is .5*b*h.
But I don't have the height. And all the formulas I know for height need the base.
Split the triangle into two right triangles each with bases equal to $\displaystyle \frac{1}{2}x$ and then use the pythagorean theorem to represent the hieght in terms of $\displaystyle x$.