4x+3y=0
y=-4x/3
x0
α在第二象限
不妨让x=-3
y=4
则r=√（3^2+4^2)=5
cosα=-3/5
cos(π/2+α)=-sinα=-4/5
sin(π/4-α)=
=sinπ/4cosα-cosπ/4sinα
=√2/2*(-3/5)-√2/2*4/5
=-7√2*10
当l绕原点顺时针旋转45°后得角β的终边,tanβ=
tanβ
=tan(π/4+α)
=(1+tanα)/(1-tanα)
=(1-4/3)/(1+4/3)
=-1/7