P
即x=-4,y=3
则r=√(x²+y²)=5
所以sina=y/r=3/5
cosa=x/r=-4/5
原式=cos(4π+a-4π)sina/(cosπ*sin(9π/2-4π)
=cosasina/(cosπ*sin(π/2)
=(-12/25)/(-1*1)
=12/25