cos(π/4+a)cos(π/4-a)=1/4 cos(π/4+a)cos(π/4-a)=1/2[cos(π/4+a+π/4-a)+cos(π/4+a-π/4+a)]=1/2[cos(π/2)+cos2a]=1/4 cos2a=1/2sina^4+cosa^4=(sina^2+cosa^2)^4-2sina^2+cosa^2=1-1/2(sin2a)^2=1-1/2[1-(co...