On the right side, [itex](sin(\theta)- cos(\theta))^2= sin^2(\theta)- 2sin(\theta)cos(\theta)+ cos^2(\theta)[/itex]
[itex]= 1- 2sin(\theta)cos(\theta)[/itex]
so [itex]1- (sin(\theta)- cos(\theta))^2= 2sin(\theta)cos(\theta)[/itex]
Do you recognize that as [itex]sin(2\theta)[/itex]?