设第三边的长度为 x .x^2 = 500^2+ 200^2 - 2×500×200cos130°= 500^2+ 200^2 + 2×500×200×cos50° = 250000 + 40000 + 200000× 0.6428= 290000 + 128557.5= 418557.5x ≈ 647 mm