g. cos2x - 5sinx - 3 = 0
<=> 1 - 2sin^2x - 5sinx - 3 = 0
<=> 2sin^2x + 5sinx + 2 = 0
<=> (sin2x + 2)(2sinx + 1) = 0
<=> 2sinx + 1 = 0 (Do sin2x + 2 > 0, với mọi x)
<=> sinx = -1/2
<=> sinx = sin(-π/6)
<=> x = -π/6 + k2π
hoặc x = 7π/6 + k2π, k nguyên

h. cos2x + cosx + 1 = 0
<=> 2cos^2x - 1 + cosx + 1 = 0
<=> 2cos^2x + cosx = 0
<=> cosx(2cosx + 1) = 0
<=> cosx = 0
hoặc cosx = -1/2
(+) cosx = 0
<=> x = π/2 + kπ, k nguyên
(+) cosx = -1/2
<=> cosx = cos2π/3
<=> x = 2π/3 + k2π
hoặc x = -2π/3 + k2π, k nguyên

i. 2sin^2(2x) - 3sin2x.cos2x + cos^2(2x) = 2
<=> 2sin^2(2x) - 3sin2x.cos2x + cos^2(2x) = 2sin^2(2x) + 2cos^2(2x)
<=> cos^2(2x) + 3sin2x.cos2x = 0
<=> cos2x(cos2x + 3sin2x) = 0
<=> cos2x = 0
hoặc cos2x + 3sin2x = 0
(+) cos2x = 0
<=> 2x = π/2 + kπ
<=> x = π/4 + kπ/2, k nguyên
(+) cos2x + 3sin2x = 0
<=> 3sin2x = -cos2x
<=> 3tan2x = -1 (cos2x = 0 không phải là nghiệm của pt)
<=> tan2x = -1/3
<=> 2x = arctan(-1/3) + kπ
<=> x = 1/2.(arctan(-1/3)) + kπ/2, k nguyên

k. sin^2x + 2sinx.cosx - 2cos^2x = 1
<=> sin^2x + 2sinx.cosx - 2cos^2x = sin^2x + cos^2x
<=> 2sinx.cosx - 3cos^2x = 0
<=> cosx(2sinx - 3cosx) = 0
<=> cosx = 0
hoặc 2sinx - 3cosx = 0
(+) cosx = 0
<=> x = π/2 + kπ, k nguyên
(+) 2sinx - 3cosx = 0
<=> 2sinx = 3cosx
<=> tanx = 3/2
<=> x = arctan3/2 + kπ, k nguyên

Tim he so cua so hang chua ×31 trong khai trien