Solve cos(x + pi) = 1/2 over the interval [pi/2, pi] pi/2 (2pi)/3 (3pi)/4 (5pi)/6

The perimeter of ? cde is 55 cm. a rhombus dmfn is inscribed in this triangle so that vertices m, f, and n lie on the sides cd , ce , and de respectively. find cd and de if cf=8 cm and ef=12 cm. the answer is not cd=20 and de=15. i have tried that one, it does not work.