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So we know that  isosceles triangles have 2 sides of equal lengths and a third side. also, two angles are equal with the third is not. since the sides meet at an angle of 40, we know that the missing angle must be 180 - 40 - 40 = 100 degrees.we also know one side (c) = 18 inches. so since we have an angle of 100 degrees across from a side length (18), we can find the other side lengths (a and b)a/sina = b/sinb = c/sinc    (a/b/c are sides, a/b/c are angles)18/sin100 = b/sin40 so b=11.7 inches so a is also a=11.7 inches