=11at(t+k+p)-9b(t+k+p)=(t+k+p)(11a-9b),uy,yuy<,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,>
=11a(t+k+p)-9b(t+k+p)=(t+k+p)(11a-9b).
sin^2A+cos^2A=1, cos2a=cos^2A-sin^2A.