7:

A+B

\(=x^4-2xy+y^2+y^2+2xy+x^2+1\)

=x^4+2y^2+x^2+1

A-B

=x^4-2xy+y^2-y^2-2xy-x^2-1

=x^4-4xy-x^2-1

5:

a: =8x^2-4x^2=4x^2

b: =(5-7)*x^2y^3z^3=-2x^2y^3z^3

c: =(3+2-1/3-1/2-1/6)*x^2y^2

=4x^2y^2

13 I wish I could help her with her business

14 The children said that they were waiting for the school bus

15 The teacher asked his students to listen to her and not to make a noise

16 My sister studied hard so she completed her exam successfully

17 She didn't decide what to wear to the party 

18 She was tired so she went home yesterday

19 He has studied English for four years

\(4\sin^22x-4\cos2x-1=0\)

\(\Leftrightarrow4\left(1-\cos^22x\right)-4\cos2x-1=0\)

\(\Leftrightarrow4-4\cos^22x-4\cos2x-1=0\)

\(\Leftrightarrow-4\cos^22x-4\cos2x+3=0\)

\(\Leftrightarrow\left[{}\begin{matrix}cos2x=\dfrac{1}{2}\\cos2x=\dfrac{-3}{2}\left(L\right)\end{matrix}\right.\Leftrightarrow\cos2x=\dfrac{1}{2}\Leftrightarrow\left[{}\begin{matrix}2x=\dfrac{\pi}{3}+k2\pi\\2x=-\dfrac{\pi}{3}-k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k\pi\\x=-\dfrac{\pi}{6}-k\pi\end{matrix}\right.\left(k\in Z\right)\) 

Chọn A

4.(1-cos²2x) - 4cos2x -1 =0

-4cos²2x - 4cos2x + 3 = 0

bấm máy tính giải phương trình bậc 2

cos x = \(\dfrac{1}{2}\) => cos x = cos \(\dfrac{\pi}{3}\)  => x = \(\dfrac{\pi}{3}\) + k2\(\pi\)

                                                    x = - \(\dfrac{\pi}{3}\)+ k2\(\pi\)

cos x = \(\dfrac{-3}{2}\) ( vô nghiệm)

83.13 : 17= 1079 : 17 =  63\(\dfrac{8}{17}\)