Chọn A.

Ta có: A = cos²x.cot²x + 3cos²x - cot²x + 2sin²x

=(  cos²x.cot²x - cot²x) + (2sin²x  + 2cos²x) + cos²x

= cot²x( cos²x - 1) + 2 + cos²x

= - cot²x. sin²x + 2 + cos²x

= -cos²x + 2 + cos²x = 2

a: \(VT=\dfrac{cot^2x}{1+cot^2x}\cdot\dfrac{1+tan^2x}{tan^2x}\)

\(=\dfrac{cot^2x}{\dfrac{1}{sin^2x}}\cdot\dfrac{\dfrac{1}{cos^2x}}{tan^2x}\)

\(=\dfrac{cot^2x}{tan^2x}\cdot\dfrac{1}{cos^2x}:\dfrac{1}{sin^2x}\)

\(=\dfrac{cot^2x}{tan^2x}\cdot\dfrac{sin^2x}{cos^2x}\)

\(=cot^2x\)

\(VP=\dfrac{tan^2x+cot^2x}{1+tan^4x}=\dfrac{\dfrac{sin^2x}{cos^2x}+\dfrac{cos^2x}{sin^2x}}{1+\dfrac{sin^4x}{cos^4x}}\)

\(=\dfrac{sin^4x+cos^4x}{sin^2x\cdot cos^2x}:\dfrac{cos^4x+sin^4x}{cos^4x}\)

\(=\dfrac{sin^4x+cos^4x}{sin^2x\cdot cos^2x}\cdot\dfrac{cos^4x}{cos^4x+sin^4x}=\dfrac{cos^2x}{sin^2x}=cot^2x\)

=>VT=VP

b:

\(\dfrac{tan^2x-cos^2x}{sin^2x}+\dfrac{cot^2x-sin^2x}{cos^2x}\)

\(=\dfrac{\left(\dfrac{sinx}{cosx}\right)^2-cos^2x}{sin^2x}+\dfrac{\left(\dfrac{cosx}{sinx}\right)^2-sin^2x}{cos^2x}\)

\(=\dfrac{sin^2x-cos^4x}{cos^2x\cdot sin^2x}+\dfrac{cos^2x-sin^4x}{sin^2x\cdot cos^2x}\)

\(=\dfrac{sin^2x+cos^2x-cos^4x-sin^4x}{cos^2x\cdot sin^2x}\)

\(=\dfrac{1-\left(cos^2x+sin^2x\right)^2+2\cdot cos^2x\cdot sin^2x}{cos^2x\cdot sin^2x}\)

\(=\dfrac{2\cdot cos^2x\cdot sin^2x}{cos^2x\cdot sin^2x}=2\)

A =  cos 6 x  + 3 sin 2 x . cos 2 x  + 2 sin 4 α .  cos 2 x  +  sin 4 α

=  cos 6 x  + 3.(1 -  cos 2 x ) cos 4 x  + 2 sin 4 α . cos 2 x  +  sin 4 α

=  cos 6 x  + 3 cos 4 x  - 3 cos 6 x  + 2 sin 4 α  - 2 sin 6 x  +  sin 4 α

= -2.( cos 6 x  +  sin 6 x ) + 3  cos 4 x  + 3 sin 4 α

= -2.( cos 6 x  +  sin 6 x ) + 3.( cos 4 x  +  sin 4 α ) = 1

Vậy biểu thức A không phụ thuộc vào x.

Ta có:

Vậy giá trị của biểu thức  không phụ thuộc vào x.

\(\frac{1}{sin2a}=\frac{sina}{sina.sin2a}=\frac{sin\left(2a-a\right)}{sina.sin2a}=\frac{sin2a.cosa-cos2a.sina}{sina.sin2a}\)

\(=\frac{sin2a.cosa}{sina.sin2a}-\frac{cos2a.sina}{sina.cos2a}=\frac{cosa}{sina}-\frac{cos2a}{sin2a}=cota-cot2a\)

Áp dụng vào bài toán:

\(A=\frac{1}{sin2y}+\frac{1}{sin2\left(2y\right)}+\frac{1}{sin2\left(4y\right)}-coty+cot8y\)

\(=coty-cot2y+cot2y-cot4y+cot4y-cot8y-coty+cot8y\)

\(=0\)

\(B=\frac{1}{sin2\left(2x\right)}+\frac{1}{sin2\left(2x\right)}+\frac{1}{sin2\left(8x\right)}-cot2x+cot16x\)

\(=cot2x-cot4x+cot4x-cot8x+cot8x-cot16x-cot2x+cot16x\)

\(=0\)

Đáp án: C

Ta có:

A = (1 -  sin 2 x ) c o t 2 x  + (1 -  c o t 2 x ) =  c o t 2 x  -  sin 2 x . c o t 2 x  + 1 -  c o t 2 x

Chọn A.

Chọn A.

Ta có:

Chọn A.

Ta có C = (1-sin²x) cot²x + 1 - cot²x.

= (1 - sin²x - 1)  cot²x + 1

= -sin²x.cot²x + 1 = -cos²x + 1 = sin²x.