4sin3x. sin5x. sin8x=sin6x

x∈[-:2; 7:2]

4sin3x·sin5x·sin8x=sin6x; x∈[-π/2; 7π/2] ⇔ x∈[-π/2; 3π+π/2]

4sin3x·sin5x·s-n8x - 2sin3x·cos3x = 0

sin3x (2sin5x·sin8x - cos3x) = 0

sin3x=0 ⇒ 3x = πk ⇒ x = π/3·k ⇒

⇒ x={0; π/3; 2π/3; π; 4π/3; 5π/3; 2π; 7π/3}

2sin5x·sin8x - cos3x = 0

sinα·sinβ = 1/2[cos (α-β) - cos (α+β) ] ⇒

(cos3x - cos13x) - cos3x = 0 ⇒

cos13x=0 ⇒ 13x = + / - π/2 + 2πn; n ∈Z

x = + / - π/26 + 2πn/13; n∈ Z

a) - π / 2 ≤ - π/26 + 2πn/13≤7π/2

b) - π/2 ≤ π/26 + 2πn/13≤7π/2

n находите сами!