Question

已知3cosa-sina=1,求sinacosa的值

Answer 1

3cosa-sina=1

9cos²a-6cosasina+sin²a=1

8cos²a-6cosasina+1=1

cosa(4cosa-3sina)=0

coaa=0或4cosa=3sina

cosa=0

sinacosa=0

4cosa=3sina(1)

3cosa-sina=1(2)

(1)*3-(2)*4

4sina=9sina-4

5sina=4

sina=4/5

cosa=3/5

sinacosa=3/5*4/5=12/25