a^2=b^2+c^2-2×b×c×cosA
BC^2 = 80^2 + 120^2 - 2 × 80 × 120 × cos72° = 6400 + 14400 - 19200 × cos72° = 20800 - 19200 × cos72°
BC =
a^2=b^2+c^2-2×b×c×cosA
BC^2 = 80^2 + 120^2 - 2 × 80 × 120 × cos72° = 6400 + 14400 - 19200 × cos72° = 20800 - 19200 × cos72°
BC =