sina=√5/5,a属于（0,90°）,
所以cos a=2√5/5,
tana= sina/cosa=1/2.
tanb=1/3,
tan2b=2 tanb/(1- tan²b)=3/4,
tan(a+2b)=( tana + tan2b)/(1- tana +tan2b)
=2.