Question

已知sina+cosa=1/3,则sinacosa+tana-1/(3cosa)=

Answer 1

sina+cosa=1/3,故两边除以cosa

tana+1=1/(3cosa),tana-1/(3cosa)=-1

sina+cosa=1/3,两边平方

1+2sinacosa=1/9,故sinacosa=-4/9

故sinacosa+tana-1/(3cosa)=-13/9