Mohon bantuannnya kakak :)

besar sudut A
a^2 = b^2 + c^2 - 2.b.c.cos A
20^2 = 13^2 + 21^2 - 2. 13.21.Cos A
400 = 169 +441-546.Cos A
400 = 610 -546 Cos A
400-610 = -546 Cos A
-210 = -546 Cos A
cos A = -210/-546
Cos A = 210/546 = 30/78= 5/13