cosa=13/20+(5/12)sina,①
∴1=sin^a+[13/20+(5/12)sina]^
=(169/144)sin^a+(13/24)sina+169/400,
(169/144)sin^a+(13/24)sina-231/400=0,
繁!
△=169/576+169*231/14400
=169*256/14400
=169*4/225,
sina=[-13/24土26/15]/(169/72)
=(-39土624/5）/169
=（-195土624）/845
=429/845,或-819/845,
代入①,求出cosa,
再根据两者的符号确定a所在象限,利用反三角函数表示a.
剩下部分留给您练习.