\(\dfrac{1}{5+2\sqrt{6}}-\dfrac{1}{5-2\sqrt{6}}=\dfrac{5-2\sqrt{6}-5-2\sqrt{6}}{5^2-\left(2\sqrt{6}\right)^2}=\dfrac{-4\sqrt{6}}{25-24}=-4\sqrt{6}\)

\(=\dfrac{5-2\sqrt{6}-5-2\sqrt{6}}{\left(5+2\sqrt{6}\right)\left(5-2\sqrt{6}\right)}=\dfrac{-4\sqrt{6}}{25-24}=-4\sqrt{6}\)

\(\left(\dfrac{6-2}{1-3}-\dfrac{5}{5}\right):\dfrac{1}{5-2}\)

\(=\left(\dfrac{4}{-2}-1\right):\dfrac{1}{3}\)

\(=\left(-2-1\right):\dfrac{1}{3}\)

\(=-3.3\)

\(=-9\)

\(=\left(\dfrac{4}{-2}-1\right):\dfrac{1}{3}\)

=(-3)*3

=-9

cái đó là toán lớp 5 nha bẩm lộn ấ

a, 5/10 - 2/15 = 1/2 - 2/15 = 15/30 - 4/30 = 11/30

b,5/20 - 1/6 = 1/4 - 1/6 = 3/12 - 2/12 = 1/12

c,6/18 - 6/24 = 1/3 - 1/4 = 4/12 - 3/12 = 1/12 

d,5/9 - 3/12 = 5/9 - 1/4 = 20/36 - 9/36 = 11/36 
chắc là đúng á , tại lớp 8 rồi ko nhớ lắm , mà vote mình nhá

B = 1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7

B = 1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7

B = 1 - 1/7

B = 6/7 

\(B=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=1-\frac{1}{7}\)

\(=\frac{6}{7}\)

\(=\dfrac{x-5+3x+15-2x-6}{\left(x-5\right)\left(x+5\right)}\cdot\dfrac{x+5}{2}\)

\(=\dfrac{2x+4}{x-5}\cdot\dfrac{1}{2}=\dfrac{x+2}{x-5}\)

1:

\(A=\sqrt{x^2+\dfrac{2x^2}{3}}=\sqrt{\dfrac{5x^2}{3}}=\left|\sqrt{\dfrac{5}{3}}x\right|=-x\sqrt{\dfrac{5}{3}}\)

2: \(=\left(\dfrac{\sqrt{100}+\sqrt{40}}{\sqrt{5}+\sqrt{2}}+\sqrt{6}\right)\cdot\dfrac{2\sqrt{5}-\sqrt{6}}{2}\)

\(=\dfrac{\left(2\sqrt{5}+\sqrt{6}\right)\left(2\sqrt{5}-\sqrt{6}\right)}{2}\)

\(=\dfrac{20-6}{2}=7\)

1/3 + 1/5 - 1/4 = 17/60

7/8 - (1/4 + 2/5) = 9/40

5/11 - (3/5 - 6/11) = 2/5

17/60
9/40
2/5