tana-6分之tana=-1
tana/(tana-6)=-1
tana=-tana+6
tana=3
sin平方a-3sinacosa+4cos平方a
=(sin²a-3sinacosa+4cos²a)/(sin²a+cos²a)
=(tan²a-3tana+4)/(tan²a+1)
=(9-9+4)/(9+1)
=4/10
=2/5
3cosa+4sina分之2cosa-3sina
=(2cosa-3sina)/(3cosa+4sina)
=(2-3tana)/(3+4tana)
=(2-9)/(3+12)
=-7/15