\(8,=\left(2x-3\right)\left(2x+3\right)\\ 9,=\left(1-5a^2\right)\left(1+5a^2\right)\)

8) \(-9+4x^2=\left(2x\right)^2-3^2=\left(2x-3\right)\left(2x+3\right)\)

9) \(1-25a^4=1-\left(5a^2\right)^2=\left(1-5a^2\right)\left(1+5a^2\right)\)

13 This museum isn't visited often by tourists

14 All this cheese is bought in France every year

15 I wasn't invited to their New Year party

16 Is mineral water sold in plastic bottle

13:

N S R I

\(i=i'\Leftrightarrow i'=40^o\)

14 + 15 + 16 (.Lý thuyết SGK nha bn!)

1.B

2.A

3.C

4.D

5.B

6.D

PTHH: 3Cu + 8HNO3 ----> 3Cu(NO3)2 + 2NO + 4H2O (1)
0,45mol 0,3 mol
CuO + 2HNO3 ---------> Cu(NO3)2 + H2O

Ta có n NO = 6,72/22,4 = 0,3 mol
Theo pthh (1): n Cu = 0,45 mol
=> m Cu = 0,45 . 64 = 28,8 g
=> m CuO = 30 - 28,8 = 1,2 g

n NO = 0,3 mol --> n Cu = 0,3*3/2 = 0,45 mol --> m Cu=28,3 gam --> m CuO = 1,2 gam

Câu 20: \(y=\frac{\sin x+2\cdot cosx+1}{\sin x+cosx+2}\)

=>\(y\left(\sin x+cosx+2\right)=\sin x+2\cdot cosx+1\)

=>\(\sin x\left(y-1\right)+cosx\left(y-2\right)+2y-1=0\)

=>\(\sin x\left(y-1\right)+cosx\left(y-2\right)=-2y+1\) (1)

Để phương trình (1) có nghiệm thì \(\left(y-1\right)^2+\left(y-2\right)^2\ge\left(-2y+1\right)^2\)

=>\(2y^2-6y+5\ge4y^2-4y+1\)

=>\(-2y^2-2y+4\ge0\)

=>\(y^2+y-2\le0\)

=>(y+2)(y-1)<=0

=>-2<=y<=1

=>Chọn B

Câu 21: \(cosx-\sqrt3\cdot\sin x=0\)

=>\(\frac12\cdot cosx-\frac{\sqrt3}{2}\cdot\sin x=0\)

=>\(\sin\left(\frac{\pi}{6}-x\right)=0\)

=>\(\frac{\pi}{6}-x=k\pi\)

=>\(x=\frac{\pi}{6}-k\pi\)

=>Chọn A