用a代替x=4ty=-3t则r=√(x²-y²)=5|t|sina=y/r=-3t/5|t|cosa=x/r=4t/5|t|所以t>0,sina=-3/5,cpsa=4/5t<0,sina=3/5,cpsa=-4/5所以t>0,原式=-2/5t<0,原式=2/5
