cos2(x+y)−cos2(x−y), neye eşittir ?
cos(x+y)=cosx.cosy−sinx.siny oldugundan
[cos(x+y)−cos(x−y)][cos(x+y)+cos(x−y)] yerlerine koyarsak
[−2sinx.siny][2cosx.cosy]=−sin2x.sin2y olur