Question

Two sides of a parallelogram measure 15 cm and 20 cm, and one of the diagonals measures 19 cm. What are the measures of the angles of the parallelogram to the nearest degree?

Answer 1

first, to get the measure <A, use the cosine rule :
cos<<A = (AB^2 + AD^2 - BD^2)/(2*AB*AD)

Answer 2

so,
cos<A = (20^2 + 15^2 - 20^2)/(2*20*15)