2+4+6+...+2n=870
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
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!
![](https://rs.olm.vn/images/avt/0.png?1311)
https://olm.vn/hoi-dap/question/212460.html
vô đây xem nha
![](https://rs.olm.vn/images/avt/0.png?1311)
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
\(2\left(1+2+3+...+n\right)=870\)
\(\leftrightarrow2.\frac{n\left(n-1\right)}{2}=870\)
\(\leftrightarrow n\left(n-1\right)=870=30.29\)
\(\rightarrow n=30\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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 $x=29$.
![](https://rs.olm.vn/images/avt/0.png?1311)
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
![](https://rs.olm.vn/images/avt/0.png?1311)
\(C=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{870}\)
\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{29.30}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{29}-\frac{1}{30}\)
\(=1-\frac{1}{30}\)
\(=\frac{29}{30}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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:
\(P=78+\left|78-129\right|+\left(-29\right)\)
\(\Rightarrow P=78+\left[-\left(78-129\right)\right]+\left(-29\right)\)
\(\Rightarrow P=78+\left(-78\right)+129+\left(-29\right)\)
\(\Rightarrow P=100\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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
2+4+6+...+2n = 870
<=> 2(1+2+3+...+n)=870
<=> 1+2+3+...+n = 870:2 = 435
<=>\(\frac{n\left(n+1\right)}{2}=435\)
<=> n(n+1) = 435.2 = 870
=> n =29
ko có số 2n đó