<span>Sin 3x + sin x = sin2x ( в левой части уравнения сумма синусов)
2Sin2x Cosx - Sin 2x = 0
Sin2x(2Cosx -1) = 0
Sin2x = 0 или 2Cosx -1 = 0
2x = n</span>π, n ∈Z Cosx = 1/2
x = nπ/2, n ∈ Z x = +-arcCos1/2 + 2πk , k ∈ Z
x = +- π/3 + 2πk , k ∈ Z
a(1) = 5
a(2) = 11
a(7) -?
S(7) -?
d = a(2) - a(1)
d = 11 - 5 = 6
a(n) = a(1) +d(n-1)
a(7) = a(1) +d * 6
a(7) = 5 + 6*6 = 5+36 = 41
S(n) = [a(1) + a(n)] / 2 * n
S(7) = [a(1) + a(7)] / 2 * 7
S(7) = (5 + 41) / 2 * 7 = 46 / 2 * 7 = 23 * 7 = 161