5a-(3a-(2a-4))=5a-(3a-2a+4)=5a-3a+2a-4=4a-4=4(a-1)
Sinx+sin3x=4cos²x
2sin(x+3x)/2×cos(x-3x)/2=4cos²x
2sin2xcosx=4cos²x
2×2×sinx×cosx×cosx=4cosx²
4cos²xsinx-4cos²x=0
4cos²x(sinx-1)=0
4cos²x=0 sinx-1=0
cosx=0 sinx=1
x=
+πn, n∈Z x=
+2πn<span>, n</span>∈Z
(27⁴)³ ÷ (9²)⁸ = 27¹² ÷ 9¹⁶ = 3³⁶ ÷ 3³² = 3³⁶ ⁻ ³² = 3⁴ = 81.