Diket: segitiga ABC cos(B+C)=9/40. Jika pnj sisi AC = 10 cm dan AB= 8cm, maka pjn sisi BC adl

Cos (B+C)= 9/40

Cos A = cos (180-(B+C))
= -cos (B+C)
= -9/40

BC^2= AB^2+AC^2-2.AB.AC.cosA
=8^2+10^2-(2)(8)(10)(-9/40)
=64+100+36
=200

BC= √200=10√2