由三角形面积公式:S = 1/2bc * sinA (1)
由余弦定理:a² = b² + c² - 2bccosA (2)
已知:x09x09S=a²-(b-c)² (3)
由(1)(3)得:
a² - b² - c² + 2bc = 1/2bcsinA (4)
将(2)带入(4)得:
- 2bccosA + 2bc = 1/2bcsinAx09
化简得:
4(1 - cosA) = sinAx09x09x09
4(1 - (1 - 2sin²(A/2) ) = 2sin(A/2)cos(A/2)
8 sin²(A/2) = 2sin(A/2)cos(A/2)
右边除到左边,即:tan(A/2) = 1/4