Gọi số cần tìm là ab (a khác 0; a, b là chữ số)

Ta có:

ab = b x b +a

=> 10 x a +b = b x b +a

=> 9 x a = b x [b-1]

Vì b , b-1 là hai số tự nhiên liên tiếp nên a và 9 là hai số tự nhiên liên tiếp =>a = 8

=> b x [b-1] = 72 => b= 9

Vậy số cần tìm là 89

\(\dfrac{x+5}{2005}+\dfrac{x+6}{2004}+\dfrac{x+7}{2003}=-3\\ \Rightarrow\dfrac{x+5}{2005}+1+\dfrac{x+6}{2004}+1+\dfrac{x+7}{2003}+1=0\\ \Rightarrow\dfrac{x+2010}{2005}+\dfrac{x+2010}{2004}+\dfrac{x+2010}{2003}=0\\ \Rightarrow\left(x+2010\right)\left(\dfrac{1}{2005}+\dfrac{1}{2004}+\dfrac{1}{2003}\right)=0\\ \Rightarrow x+2010=0\\ \Rightarrow x=-2010\)

b) \(A=\dfrac{3}{1.4}+\dfrac{3}{4.7}+\dfrac{3}{7.10}+...+\dfrac{3}{94.97}+\dfrac{3}{97.100}\\ =1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{94}-\dfrac{1}{97}+\dfrac{1}{97}-\dfrac{1}{100}\\ =1-\dfrac{1}{100}=\dfrac{99}{100}\)

1 A

2 D

3 C

4 C

5 D

1A 2D 3C 4C 5D

\(\overline{ab}=b^2+a\)

\(10a+b=b^2+a\)

\(9a=b^2-b\)

\(a=\dfrac{b\left(b-1\right)}{9}\)

\(b=9,a=8\)

Gọi số cần tìm là \(\overline{ab}\)

Ta có:\(\overline{ab}=b^2+a\)

Mà:\(\overline{ab}=10a+b\)

\(\Rightarrow b^2+a=10a+b\)

\(\Rightarrow b^2-b=10a-a\)

\(\Rightarrow b\left(b-1\right)=9a\)

Vì \(b\left(b-1\right)\) là \(2\) số tự nhiên liên tiếp nên \(9a\) là \(2\) số tự nhiên liên tiếp

\(\Rightarrow\left[{}\begin{matrix}a=8\\a=10\end{matrix}\right.\)

Mà \(a< 10\)

\(\Rightarrow a=8\)

\(\Rightarrow b\left(b-1\right)=9.8\)

\(\Rightarrow b=9\)

Vậy số cần tìm là \(89\)

`#3107.101107`

`8)`

\(15\cdot24-14\cdot5\left(145\right)\div5-27\\ =15\cdot24-14\cdot145-27\\ =360-2030-27\\ =-1697\)

\(\left(2x+1\right)\left(4x^2-2x+1\right)-\left(2x-1\right)\left(4x^2+2x+1\right)\)

\(=8x^3+1-\left(8x^3-1\right)=8x^3+1-8x^3+1=2\)

\(C=\left(3x+2\right)^2-\left(3x+2\right)\left(3x-2\right)-6x\)

\(=9x^2+12x+4-\left(9x^2-4\right)-6x=6x+8\)

Vậy bth phụ thuộc biến x, ko có đpcm 

Vẽ hình vuông thì cần gì compa em ơi?

a: \(\dfrac{1}{2\text{x}5}+\dfrac{1}{5\text{x}8}+...+\dfrac{1}{14\text{x}17}\)

\(=\dfrac{1}{3}\text{x}\left(\dfrac{3}{2\text{x}5}+\dfrac{3}{5\text{x}8}+...+\dfrac{3}{14\text{x}17}\right)\)

\(=\dfrac{1}{3}\text{x}\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+...+\dfrac{1}{14}-\dfrac{1}{17}\right)\)

\(=\dfrac{1}{3}\text{x}\left(\dfrac{1}{2}-\dfrac{1}{17}\right)=\dfrac{1}{3}\text{x}\dfrac{15}{34}=\dfrac{5}{34}\)

b: \(\dfrac{1}{1\text{x}5}+\dfrac{1}{5\text{x}9}+...+\dfrac{1}{17\text{x}21}\)

\(=\dfrac{1}{4}\text{x}\left(\dfrac{4}{1\text{x}5}+\dfrac{4}{5\text{x}9}+...+\dfrac{4}{17\text{x}21}\right)\)

\(=\dfrac{1}{4}\text{x}\left(1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+...+\dfrac{1}{17}-\dfrac{1}{21}\right)\)

\(=\dfrac{1}{4}\text{x}\left(1-\dfrac{1}{21}\right)=\dfrac{1}{4}\text{x}\dfrac{20}{21}=\dfrac{5}{21}\)

ừ