已知tana=3,sin^2a+sinacosa+2cos^2a
问题描述：
已知tana=3,sin^2a+sinacosa+2cos^2a
答
(sina)^2+sinacosa+2(cosa)^2
=1+ sinacosa+ (cosa)^2
= 1+ ( sina + cosa)/{ 1/cosa}
= 1+ ( tana + 1)/ {1/(cosa)^2}
= 1+ (3+1)/ (10) ( cosa = 1/√10)
= 7/5