Ta có I2x-1I >= 0 với mọi x thuộc Z

          Ix+2I >= 0 với mọi x thuộc Z

          Ix+3I >=0 với mọi x thuộc Z

Vì I2x-1I+Ix+2I+Ix+3I=5x-1

=> 5x-1 >=0 

=> I2x-1I+Ix+2I+Ix+3I=2x-1+x+2+x+3=4x+4

=> 4x+4=5x-1

<=> 4x-5x=-1-4

<=> -x=-5

<=> x=5

Vậy x=5

18: \(\left(x^2-4\right)\left(x^2+4\right)=x^4-16\)

19: \(\left(x+y\right)^2+\left(x-y\right)^2=2x^2+2y^2\)

20: \(\left(2x+3\right)^2-\left(x+1\right)^2\)

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

\(=\left(x+2\right)\left(3x+4\right)\)

vừa cho câu hỏi vừa trả lời à ?

1, \(A=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{99\cdot100}\)

giải :

\(A=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{99\cdot100}\)

\(A=\frac{2-1}{1\cdot2}+\frac{3-2}{2\cdot3}+\frac{4-3}{3\cdot4}+...+\frac{100-99}{99\cdot100}\)

\(A=\frac{2}{1\cdot2}-\frac{1}{1\cdot2}+\frac{3}{2\cdot3}-\frac{2}{2\cdot3}+\frac{4}{3\cdot4}-\frac{3}{3\cdot4}+...+\frac{100}{99\cdot100}-\frac{100}{99\cdot100}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\)

\(A=1-\frac{1}{100}\)

\(A=\frac{99}{100}\)

2, \(S=\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{100}}\)

giải :

\(S=\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{100}}\)

\(2S=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{99}}\)

\(2S-S=\left(1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{99}}\right)-\left(\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{100}}\right)\)

\(S=1-\frac{1}{2^{100}}\)

3, \(I=\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)...\left(1-\frac{1}{100}\right)\)

giải :

\(I=\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)...\left(1-\frac{1}{100}\right)\)

\(I=\frac{1}{2}\cdot\frac{2}{3}\cdot\frac{3}{4}\cdot...\cdot\frac{99}{100}\)

\(I=\frac{1\left(2\cdot3\cdot...\cdot99\right)}{\left(2\cdot3\cdot4\cdot...\cdot99\right)\cdot100}\)

\(I=\frac{1}{100}\)

3 dạng cơ bản này thôi nhé!

Tìm x, biết:

a)   x – 1/4 = 5/8 . 2/3                          

b)  x/126 = -5/9. 4/7

nhanh nhé các bạn

35.18-5.7.35

= 35.18 - 35.35

= 35. ( 18-35)

= 35. (-17)

=-595

\(=35\cdot18-35\cdot35\)

\(=35\left(18-35\right)=-595\)