Sin4x = 2cos^2(x + pi/3) - 1 Tham khảo câu trả lời từ Trợ lý ảo Lazi To solve the equation sin(4x) = 2cos^2(x + π/3) - 1, we can use the trigonometric identity sin(2θ) = 2sin(θ)cos(θ). First, let's simplify the right side of the equation using the given identity: 2cos^2(x + π/3) - 1 = 2(cos(x + π/3))^2 - 1 = 2(cos^2(x)cos^2(π/3) - sin^2(x)sin^2(π/3)) - 1 = 2(cos^2(x)(1/4) - sin^2(x)(3/4)) - 1 = (1/2)cos^2(x) - (3/2)sin^2(x) - 1 Now, substitute this expression back into the original equation: sin(4x) = (1/2)cos^2(x) - (3/2)sin^2(x) - 1 Using the double angle identity sin(2θ) = 2sin(θ)cos(θ), we can rewrite sin(4x) as: sin(4x) = 2sin(2x)cos(2x) Now, substitute this back into the equation: 2sin(2x)cos(2x) = (1/2)cos^2(x) - (3/2)sin^2(x) - 1 Next, let's use the double angle identity for cosine: cos(2θ) = cos^2(θ) - sin^2(θ) 2sin(2x)cos(2x) = (1/2)(cos(2x) + 1) - (3/2)sin^2(x) - 1 Distribute the (1/2) to the terms inside the parentheses: 2sin(2x)cos(2x) = (1/2)cos(2x) + 1/2 - (3/2)sin^2(x) - 1 Combine like terms: 2sin(2x)cos(2x) = (1/2)cos(2x) - (3/2)sin^2(x) - 1/2 Multiply both sides of the equation by 2 to eliminate the fractions: 4sin(2x)cos(2x) = cos(2x) - 3sin^2(x) - 1 Now, let's simplify the equation further: 4sin(2x)cos(2x) - cos(2x) + 3sin^2(x) + 1 = 0 Factor out a common factor of cos(2x): cos(2x)(4sin(2x) - 1) + 3sin^2(x) + 1 = 0 Now, let's focus on the first term: cos(2x)(4sin(2x) - 1) To find the values of x that satisfy this term, we have two cases to consider: Case 1: cos(2x) = 0 This occurs when 2x = π/2 + nπ or 2x = 3π/2 + nπ, where n is an integer. Solving for x, we have x = π/4 + (nπ)/2 or x = (3π/4 + nπ)/2, where n is an integer. Case 2: 4sin(2x) - 1 = 0 Solving for sin(2x), we have sin(2x) = 1/4 This occurs when 2x = π/6 + 2nπ or 2x = 5π/6 + 2nπ, where n is an integer. Solving for x, we have x = π/12 + (nπ)/2 or x = (5π/12 + nπ)/2, where n is an integer. Therefore, the solutions to the equation sin(4x) = 2cos^2(x + π/3) - 1 are: x = π/4 + (nπ)/2, (3π/4 + nπ)/2, π/12 + (nπ)/2, (5π/12 + nπ)/2, where n is an integer.