Sum, Difference, and Multiple-Angle Formulas · Easy problem

$\tfrac12\big[\cos 2x-\cos 6x\big]$

$\tfrac12\big[\cos 4x+\cos 6x\big]$

$\tfrac12\big[\cos 2x+\cos 7x\big]$

$\tfrac12\big[\cos 2x+\cos 6x\big]$