sina+cosa=1/5,两边平方可得,
sina*cosa=-12/25.
有方程式为:
x^2-1/5x-12/25=0,
(x-4/5)(x+3/5)=0,
x1=4/5,x2=-3/5.
a属于(o,180),所以有,
sina=4/5,cosa=-3/5.
tana=sina/cosa=-4/3.
2.
tan(a+b)=2/5,
(tana+tanb)/(1-tana*tanb)=2/5,
tan(b-45度）=1/4 ,
(tanb-tan45)/(1+tanb*tan45)=1/4.
解得.tana=-19/25,tanb=5/3.
tan(a+45)=(tana+tan45)/(1-tana*tan45)=(tana+1)/(1-tana)=3/22.