已知tana=-1/3求sin²a+3sinacosa-1的值

已知tana=-1/3求sin²a+3sinacosa-1的值
已知tana=-1/3求sin²a+3sinacosa-1的值

已知tana=-1/3求sin²a+3sinacosa-1的值
tana=-1/3
sina/cosa=-1/3
-3sina=cosa
cos²a+sin²a=1
9sin²a+sin²a=1
10sin²a=1
sin²a=1/10
sin²a+3sinacosa-1
=sin²a-9sin²a-1
=-8sin²a-1
=-8/10-1
=-9/5