cos(a+b)=cosacosb-sinasinbcos(a-b)=cosacosb+sinasinbcos(a+b)+cos(a-b)=2cosacosbcos(π/4+x+π/4-x)+cos(π/4+x-π/4+x)=2cos(π/4+x)cos(π/4-x)=1/2cos(π/2)+cos(2x)=1/2cos2x=1/2sin2x=±√3/2sin²2x=3...