Question

Упростите выражение cos п/7 * cos п/42 - sin п/7 * sin п/42

Answer 1

сosα*cosβ-sinα*sinβ=cos(α+β)

(cos(π/7)) * (cos(π/42)) - (sin(π/7)) * (sin(π/42))=cos(π/7+π/42)cos(7π/42)=cosπ/6=√3/2