2+4+6+...+2n=870

2(1+2+3+.....+n)=870

1+2+3+....+n=870/2=435

=> n=29

tik nhá mk sẽ mong bn may mắn trong năm mới!

(2 + 2 * n) * n : 2 = 870 => n= 29

                                       n= -30.n là số tự nhiên nên n = 29 

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vô đây xem nha

   2+4+6+........+2n=870

2.(2+4+6+.......+2n)=870

    2+4+6+........+2n=870:2

    2+4+6+....... +2n=435

=>n.(n+1):2=435

n.(n+1)=435.2

n.(n+1)=870

Vì 29.30=870=>n=29

\(2\left(1+2+3+...+n\right)=870\)

\(\leftrightarrow2.\frac{n\left(n-1\right)}{2}=870\)

\(\leftrightarrow n\left(n-1\right)=870=30.29\)

\(\rightarrow n=30\)

2+4+6+...+2n=870

=>2(1+2+3+...+n)=870

=>1+2+3+...+n=870:2

=>1+2+3+...+n=435

=>(n+1).n:2=435

=>(n+1).n=435.2

=>(n+1).n=870

=>(n+1).n=29.30

=>n=29

Lời giải:
$2+4+6+...+2x=870$

$2(1+2+3+...+x)=870$

$1+2+3+...+x=870:2$

$x(x+1):2=870:2$

$\Rightarrow x(x+1)=870=29.30$

Suy ra $x=29$.

Tìm số tự nhiên $n$ thỏa mãn: 55 . 6�n = 71280.

x = 29

a)C= (-124)+ (36 + 124 - 99 ) - ( 136 - 1 ) = (-124) +36 +124 -99 - 136 +1

= -198

b) D = { 115+[ 32 - ( 132 -5 )] } +(-25) +(-25)

= {115+[32-132+5]} +(-25) + (-25)

= {115+(-95)} + (-25)+ (-25)

= 20 +(-25) +(-25) = -30

c)F = [(123 - 17 ) - (123 + 33 ) ] - { 34 - [ 34 + ( 57 -50 ) -7 ] }

= [123 -17 -123 -33 ] - {34- [34 + 57-50 -7]}

= -50 - {34-34} = -50 - 34 +34 = -50

d) mik bo nha

e) E = (-7) +( -2020) -(-7) +2020

= [(-7)-(-7)]+[(-2020) +2020]= 0

g)B= (-2019)-( 29 - 2019) = (-2019) - 29 +2019 = -29

co moi cau d mik ko biet lam :v

\(C=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{870}\)

\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{29.30}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{29}-\frac{1}{30}\)

\(=1-\frac{1}{30}\)

\(=\frac{29}{30}\)

Bài 1:
Giải:

Ta có: \(a+b=-3\)

\(\Rightarrow a+b+c=-3+c\)

\(\Rightarrow a+\left(-5\right)=-3+c\)

\(\Rightarrow a-c=\left(-3\right)-\left(-5\right)\)

\(\Rightarrow a-c=2\)

Mà \(c+a=-4\)

\(\Rightarrow a=\left(-4+2\right):2=-1\)

\(\Rightarrow c=\left(-4\right)-\left(-1\right)=-3\)

Lại có: \(b+c=-5\)

\(\Rightarrow b+\left(-3\right)=-5\)

\(\Rightarrow b=-2\)

Vậy bộ số \(\left(a;b;c\right)\) là \(\left(-1;-3;-2\right)\)

Bài 2:

\(P=78+\left|78-129\right|+\left(-29\right)\)

\(\Rightarrow P=78+\left[-\left(78-129\right)\right]+\left(-29\right)\)

\(\Rightarrow P=78+\left(-78\right)+129+\left(-29\right)\)

\(\Rightarrow P=100\)

a) (137x25-25x37):10

=25x(137-37):10

=25x100:10

=2500:10

=250

b) 2/7x21/4-2/7:13/4

=2/7x21/4-2/7x4/13

=2/7x(21/4-4/13)

=2/7x257/52

=257/182

c)(124x237+152):(870+235x122)

=(29388+152):(870+28670)

=29540:29540

=1

a) (137 x 25 - 25 x 37 ) : 10 = (25 x (137 - 37) ) : 10

                                       = (25 x 100) : 10

                                       = 2500 : 10

                                       = 250