Triangle ABC is isosceles with AB = BC. If AC = 20 and [ABC] = 280, then find the perimeter of triangle ABC.
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My solution: Draw the altitude BP. Then PA = PC = 10. So, we have that 20*BP/2 = 280, so BP = 28. As we have that 10^2+28^2 = 884, the perimeter of ABC is 4√221 + 20.