Bài 9:

Ta có: \(a-4+\sqrt{16-8a+a^2}\)

\(=a-4+\sqrt{\left(a-4\right)^2}\)

\(=a-4+a-4\)

=2a-8

a)\(\dfrac{x^2-3}{x^2+2x\sqrt{3}+3}=\dfrac{\left(x-\sqrt{3}\right)\left(x+\sqrt{3}\right)}{\left(x+\sqrt{3}\right)^2}=\dfrac{x-\sqrt{3}}{x+\sqrt{3}}\)

\(\dfrac{x^2-2x\sqrt{15}+15}{x^2-15}=\dfrac{\left(x-\sqrt{15}\right)^2}{\left(x-\sqrt{15}\right)\left(x+\sqrt{15}\right)}=\dfrac{x-\sqrt{15}}{x+\sqrt{15}}\)

\(\dfrac{4x^2-6}{4x^2-4x\sqrt{6}+6}=\dfrac{\left(2x-\sqrt{6}\right)\left(2x+\sqrt{6}\right)}{\left(2x-\sqrt{6}\right)^2}=\dfrac{2x+\sqrt{6}}{2x-\sqrt{6}}\)

b) \(\dfrac{a^2+2a\sqrt{8}+8}{a^2-8}=\dfrac{\left(a+\sqrt{8}\right)^2}{\left(a+\sqrt{8}\right)\left(a-\sqrt{8}\right)}=\dfrac{a+\sqrt{8}}{a-\sqrt{8}}\)

\(\dfrac{9x^2-15}{9x^2-6x\sqrt{15}+15}=\dfrac{\left(3x-\sqrt{15}\right)\left(3x+\sqrt{15}\right)}{\left(3x-\sqrt{15}\right)^2}=\dfrac{3x+\sqrt{15}}{3x-\sqrt{15}}\)

\(\dfrac{a^2-2a\sqrt{7}+7}{a^2-7}=\dfrac{\left(a-\sqrt{7}\right)^2}{\left(a-\sqrt{7}\right)\left(a+\sqrt{7}\right)}=\dfrac{a-\sqrt{7}}{a+\sqrt{7}}\)

Bài 4:

a. \(\left\{{}\begin{matrix}R1=\dfrac{U1^2}{P1}=\dfrac{120^2}{40}=360\Omega\\R2=\dfrac{U2^2}{P2}=\dfrac{120^2}{60}=240\Omega\end{matrix}\right.\)

b. \(P1< P2=>\) đèn 2 sáng hơn.

c. \(U1+U2=120+120=240V=U=240V=>\) đèn sáng bình thường.

=>x+1/2+x+1/6+...+x+1/90=99,9

=>\(\left(x+x+...+x\right)+\left(\dfrac{1}{2}+\dfrac{1}{6}+...+\dfrac{1}{90}\right)=99.9\)

=>\(9x+\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)=99.9\)

=>9x+(1-0,1)=99,9

=>9x=99,9+0,1-1=100-1=99

=>x=11

Em tham khảo:

Cách ngắt nhịp của hai dòng thơ:     

Lặng rồi/ cả tiếng con ve, 

Con ve cũng mệt/ vì hè nắng oi.

2(1-2x)-5=3(x+2)

=>\(2-4x-5=3x+6\)

=>\(-4x-3=3x+6\)

=>\(-7x=9\)

=>\(x=-\dfrac{9}{7}\)

học thì học, chơi là chơi chứ

Mik thực hành nhiều lần rồi:))

...

II

1 B

2 C

3 A

4 D

5 C

6 B

III

1 Where do they live?

2 Why do you like "Tom and Jerry"

3 When do they do their homework?

4 What is that?

5 What vegetable do you like?

bn chụp ngang ik mik dùng mt 

1 B

2 C

3 A

4 D

5 C

6 B

hơi khó nhìn sai thì thôi nhá

\(m_{Na}=\dfrac{62.74,19\%}{100\%}=46\left(g\right)\)

\(n_{Na}=\dfrac{46}{23}=2\left(mol\right)\)

\(m_O=62-46=16\left(g\right)\\ n_O=\dfrac{16}{16}=1\left(mol\right)\\ =>CTHH:Na_2O\)