P102第29题 (1)y=180°-a-B 13 (2)2=42,=mn√Pn=±9√4=±18 ±18 ±18 ±12.7",m ±15.6 V3P102第29题 ( ) ( ) ( ) 15.6 3 4 18 12.7 , 2 18 3 2 , 9 4 18 3 4 , 4 1 1 1 3 1 180 , 2 2 =    =   = =  = = = = =   =   = − − = + =  =                    P m P m m P m P P P P P 