Question

已知sin a-2cos a=0 求4sin^2 a-3sin a x cos a-5cos^2 a的值

Answer 1

sin a-2cos a=0 ,则

tana=2,

则sina=2/根号5,-2/根号5

cosa=1/根号5,-1/根号5

则cos^2=1/5

4sin^2 a-3sin a x cos a-5cos^2 a

=1/cos^2 a(4tan^2a-3tana-5)

=1/cos^2 a(4*2^2-3*2-5)

=5/cos^2

=5*5

=25