[chemistry guy]

1
sint*(1/sint)=1



5
cos^2x-sin^2x
=1-sin^2x-sin^2x
1-2sin^2x

11
sec^2x/tanx=
(1/cos^2x)/(sinx/cosx)
=(1/cosx)*(1/sinx)=secx cscx

14
=(sin^2x+cos^2x)/sinx-
sin^2x/sinx
=cos^2x/sinx