diketahui ∆ABC dengan BC= 3cm, AC=4cm, dan

Aturan Sinus
BC/sin A = AC/sin B
3 cm/sin 30° = 4 cm/sin B
3 cm/(1/2) = 4 cm/sin B
6 cm = 4 cm/sin B
sin B = 4 cm/6 cm
sin B = (2/3) cm

sin B = De/Mi = (2/3) cm

Sa^2 = Mi^2 - De^2
Sa^2 = 3^2 - 2^2
Sa^2 = 9 - 4
Sa = akar(5)

cos B = Sa/Mi = akar(5)/3 = (1/3)akar(5)