13 x 7= 91 

tuy ko biết bạn đang nói cái gì nhưng nếu bạn ko k đúng cho mk thì mong bạn cũng đừng k sai nha

13 x 7  = 91 tuy không hiểu gì thì mong bạn thông cảm

bgxvcgđ

Ta có: \(\frac{1}{n^3}< \frac{1}{n^3-n}=\frac{1}{\left(n-1\right).n.\left(n+1\right)}=\frac{1}{2}.\frac{\left(n+1\right)-\left(n-1\right)}{\left(n-1\right).n.\left(n+1\right)}=\frac{1}{2}\left[\frac{1}{\left(n-1\right).n}-\frac{1}{n\left(n+1\right)}\right]\)

Áp dụng ta được: 

\(A< \frac{1}{2}\left(\frac{1}{4.5}-\frac{1}{5.6}+\frac{1}{5.6}-\frac{1}{6.7}+...+\frac{1}{19.20}-\frac{1}{20.21}\right)\)

\(=\frac{1}{2}\left(\frac{1}{4.5}-\frac{1}{20.21}\right)\)

\(< \frac{1}{2}.\frac{1}{4.5}=\frac{1}{40}\)

18576: {\(105^0\)+[2.(102+101−100−99+98+97−96−95+.........+6+5−4−3+2+1)−201]}^3

Đặt A=102+101−100−99+98+97−96−95+...............+6+5−4−3+2+1

A=(102+101−100−99)+(98+97−96−95)+........+(6+5−4−3)+2+1

A=4+4+...........+4+3

A=4.25+3

A=103

⇒18576:{1050+[2.(102+101−100−99+98+97−96−95+.........+6+5−4−3+2+1)−201]}^3

=18576:[1+(2.103)−201]^3

=18576:63

=18576:216

=86

bằng 86 bn nhé

1) \(2x\left(x-5\right)+\left(x-2\right)\left(x+3\right)=2x^2-10x+x^2+3x-2x-6=3x^2-9x-6\)

2) \(\left(2x-5\right)\left(1-x\right)-\left(x-3\right)\left(-2x\right)=2x-2x^2-5+5x+2x^2-6x=x-5\)

3) \(\left(4x-3\right)\left(4x-3\right)-\left(3x+2\right)\left(3x-2\right)=\left(4x-3\right)^2-9x^2+4=16x^2-24x+9-9x^2+4\)

\(=7x^2-24x+13\)

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

\(=\left(2x-1\right)\left(4x^2+4x+4\right)-4x^2-4=8x^3+8x^2+8x-4x^2-4x-4-4x^2-4=8x^3+4x-8\)

5) \(3x\left(2x-8\right)-\left(2-6x\right)\left(5+x\right)=6x^2-24x-10-2x+30x+6x^2=12x^2+4x-10\)

6) \(x\left(3x-18\right)-3\left(x-4\right)\left(x-2\right)+8=3x^2-18x-3x^2+6x+12x-24+8=-16\)

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

a ) 6 ⋮ n

\(\Rightarrow\)n ∈ Ư ( 6 )

\(\Rightarrow\)n ∈ { ± 1 ; ± 2 ; ± 3 ; ± 6 }

Vậy n ∈ { ± 1 ; ± 2 ; ± 3 ; ± 6 }

b ) 6 ⋮ ( n - 1 )

\(\Rightarrow\)n - 1 ∈ Ư ( 6 )

\(\Rightarrow\)n - 1 ∈ { ± 1 ; ± 2 ; ± 3 ; ± 6 }

Ta có bảng sau :

Vậy n ∈ { - 5 ; - 2 ; - 1 ; 0 ; 2 ; 3 ; 4 ; 7 }

4S=15+17+19+21+...+151+153+155

⇔4S= \(\frac{\left(15+155\right)\left[\left(155-15\right):2+1\right]}{2}\)

⇔4S=  \(\frac{170\cdot\left[140:2+1\right]}{2}\)

⇔4S=    \(\frac{170\cdot\left[70+1\right]}{2}\)

⇔4S =  \(\frac{170\cdot71}{2}\)

⇔4S= 85 * 71

⇔4S=6035

⇔S=60354

s= 60354

2,1kg nhé

Bài này chắc không phải toán lớp 3 đâu vì có số thập phân.

HOK TỐT

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