由△ABC,AB=AC=17.7,∠A=70°.
求第三边BC,用余弦定理：
BC²=17.7²+17.7²-2×17×17×cos70°
=313.29+313.29-2×313.29×0.324
=423.568
∴BC=20.58.兄台，那个cos70算错了啊，cos70°=0.342，没错啊，对不起cos70°写成0.324了。BC²=313.29+313.29-2×313.29×0.342BC=20.55