On Tue, 28 May 2013 08:15:05 -0700 (PDT), 130bcd <firstname.lastname@example.org> wrote:
>Hello, > >I have unsuccessfully tried to find the perimeter of a pentagon. The "radius" to the vertices is 130, which I tried to convert into 5 triangles but I can't find a formula for determining the length of the 3rd side of an isosceles triangle. > >The two equal length sides would, of course be 130 and the angle between them would be 72.
Try the law of cosines: c^2 = a^2 + b^2 - 2*a*b*cos(C)