Tuliskan aturan, rumus, serta cara pengerjaan soal pada gambar berikut no.21 :

BC" = AB" + AC" - 2 AB. AC. Cos A

BC" = 8" + 10" - 2.8.10 Cos 120

BC " = 64 + 100 - 160 (-1/2)

BC" = 164 +80

BC" = 244

BC = akar (244)

BC = akar (4. 61)

Bc = 2 akar 61 (A)

Keterangam tanda " = kuadrat