实事评论员
春芽
共回答了20个问题采纳率:90% 举报
(1)由题意可得 [1/4]×[2π/ω]=-[π/3]+[5π/6],∴ω=1,故f(x)=2sin(x+∅).
又f(-[π/3] )=2sin(-[π/3]+∅)=2,∴sin(-[π/3]+∅)=1,∴(-[π/3]+∅)=2kπ+[π/2],k∈z,
∴∅=2kπ+[5π/6],根据 0<φ<2π,∴∅=[5π/6].
(2)在△ABC中,f(B)=-2=2sin(B+[5π/6]),∴sin(B+[5π/6])=-1,根据 [5π/6]<B+[5π/6]<[11π/6],
∴B+[5π/6]=[3π/2],B=[2π/3].由余弦定理可得 b2=a2+c2-2accosB
=16+4-2×4×2(-[1/2])=28,故 b=2
7.
1年前
6