典例剖析例2、已知角α的终边经过点P0(-3,-4),求角α 的正弦、余弦。解：设角α 的终边与单位圆的交点为y P(x,y),过P作PM⊥x轴于M,过P0作P0 M0 ⊥ x轴 . 显然Rt OMP∽ Rt OM0P0 且
M0
O A(1,0) x sin y | MP | P(x,y)cos x | OM |
M
α
| OP 0 |
( 3) 2 ( 4) 2 5 4 | MP | | M 0 P0 | 5 | OP0 | | OP |
P0(-3,-4)
| OM | | OM 0 | 3 | OP | | OP0 | 5