Преобразовать в сумму cos7Π/12 *cosΠ/12

cos[latex] \frac{7pi}{12} [/latex]*cos[latex] \frac{pi}{12} [/latex]=[latex] \frac{cos( \frac{7pi}{12}+ \frac{pi}{12} )+cos( \frac{7pi}{12}- \frac{pi}{12} )}{2} [/latex]=[latex] \frac{cos \frac{8pi}{12}+cos \frac{6pi}{12} }{2} [/latex]=[latex] \frac{cos \frac{2pi}{3}+cos \frac{pi}{2} }{2} [/latex]=[latex] \frac{-0.5+0}{2} [/latex]=-0.25