高一数学题已知tana(a代表阿尔法)=3,计算(1)4sina-2cosa/5cosa+3sina.(2) (sina

(4sin a-2cos a)/(5cos a+3sin a)
分子分母同除以cos a,得：
原式=(4tan a-2)/(5+3tan a)=(12-2)/(5+9)=5/7
(sin a+cos a)^2
=(sin a+cos a)^2/[(sin a)^2+(cos a)^2]
=[(sin a)^2+(cos a)^2+2sin a*cos a]/[(sin a)^2+(cos a)^2]
=[(tan a)^2+1+2tan a]/[(tan a)^2+1]
=(9+1+6)/(9+1)
=8/5