sin^2a+cos^2a=1
sin^2a+9/4sin^a=1
sin^2a=4/13
（3sinα-2cosα）^2=0
9sin^2a-12sinacosa+4cos^2a=0
5sin^a-12sinacosa+4=0
sinacosa=6/13
sin²α-2sinαcosα-2=-34/13