tn45=tg(10+35)=tg10+tg35/(1-tg10*tg35)
(1+tan10)(1+tan35)
=1+tg10*tg35+tg10+tg35
=1+tg10*tg35+tg45(1-tg10*tg35)
=2
实际上对任意a+b=45皆可如此证明
因为tan45°=(tanA+tanB)/(1-tanAtanB)=1,所以tanA+tanB=1-tanAtanB,所以(1+tanA)(1+tanB)=1+tanA+tanB+tanAtanB=1+1-tanAtanB+tanAtanB=2.