1,sina+cosa=1/5 a属于(o,180) 则tana=? 2tan(a+b)=2/5 tan(b-45度）=

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

两边同时平方，然后就出来了

sina+cosa=1/5 a属于(o,180)
sin^2a+cos^2a=1
sina=4/5
cosa=-3/5
tana=-4/3
2tan(a+b)=2/5
tan(a+b)=1/5
tan(b-45）=1/4
tan(a+45)=tan[(a+b)-(b-45)]
=[tan(a+b)-tan(b-45)]/[1-tan(a+b)tan(b-45)]
=(-1/20)/(19/20)
=-1/19