Question

·请教·几道关于解三角形的题··带过程哈

Answer 1

1.

设B = 60°,S = (1/2)·ac·sinB = 10√3 ,ac = 40 ,b^2 = a^2 + c^2 - 2ac·cosB = a^2 + c^2 - ac ,b^2 + 3ac = (a + c)^2 = (20 - b)^2 ,

b^2 + 120 = (20 - b)^2 ,b = 7 ,a + c = 13 ,ac = 40 ,a = 8 ,c = 5 或a = 5 ,c = 8

2.

3/5 = cosA ,sinA = 4/5 ,S = (1/2)·(4/5)·bc = 14 ,bc = 35 ,b-c = 2 ,解一元二次方程 ,b = 7 ,c = 5

3.

由余弦定理 ,cosB = (a^2 + c^2 - b^2)/2ac = (a^2 + 4a^2 - a·2a)/4a^2

= 3/4 .