锐角ab互余
b=90°-a
tanb=tan(90°-a)
tanb=cota
tanatanb
=tana*cota
=1
(tana)^2+(tanb)^2
=(tana)^2+(tanb)^2-2tanatanb+2tanatanb
=(tana-tanb)^2+2tanatanb
=2^2+2*1
=4+2
=6