Question 402974
Do you know the height (ie the length that's perpendicular to the unknown base)? If you knew that value, you can easily find half the base (which can be doubled to get the base).



Say that half the base is equal to 'x' and the height is 'h'.



By the pythagorean theorem, we can say {{{x^2+h^2=8^2}}} or {{{x^2+h^2=64}}}. Draw a picture so you can see what's going on.



Now solve for x to get {{{x=sqrt(64-h^2)}}} (note: we're only considering positive numbers)



Now multiply both sides by 2 to get the length of the entire base {{{2x=2*sqrt(64-h^2)}}} 



So the base is {{{2*sqrt(64-h^2)}}} units. From here, just plug in the given height (if there is one) 'h' to evaluate the expression.



Once you've finally figured out the base, you can use that and the height to compute the area.