a+b=3 π/4,则（1-tan a)(1-tan b)=
问题描述：
a+b=3 π/4,则（1-tan a)(1-tan b)=
答
tan(a+b)=-1
(tana+tanb)/(1-tanatanb)=-1
tana+tanb=tanatanb-1
(1-tan a) (1-tan b)
=1-tana-tanb+tanatanb
=1+tanatanb-(tana+tanb)
=1+tanatanb-(tanatanb-1)
=2