1）求Sin&=-3/5与Cos&=-4/5tan2&=2tan&/(1-tan&*tan&)=(3/2)/(1-9/16)=24/7（2）tan(2&-π/4)=(tan2&-tanπ/4)/(1+tan2&tanπ/4)=(24/7-1)/(1+24/7)=17/31
