向量a=(α,β) b=(λ,μ) 若平行 则α*μ+β*λ=0 tanX=sinX/cosX=-3/2
f(x)=(sinx+cosx,1/2)*(cosx,-1)=sinxcosx+(cosx)^2-1/2=1/2sin2x+1/2cos2x=√2/2sin(2x+π/4)
2x+π/4∈(-3π/4,π/4) f(x)∈(-1/2,1/2)