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(123+345) + ( 456- 123)- [2017 -(-345)]
=123+345+456-123-2017-345
=(123-123)+(345-345)+(456-2017)
=0+0-1561
=-1561
![](https://rs.olm.vn/images/avt/0.png?1311)
Trả lời :
a, X + 345 = 456
X = 456 - 345
X = 111
b, 3*X +15 = 45
3*X = 45 - 15
3*X = 30
X = 30 : 10
X = 3
c, 2*[ x-11] = 200
x-11 = 200 : 2
x-11 = 100
x = 100 + 11
x = 111
d, 16+2*X = 30
2*X = 30 - 16
2*X = 14
X = 14 : 2
X = 7
#Study_well..
a) x + 345 = 456
x = 456 - 345
x = 111
b) 3.x + 15 = 45
3.x = 45 - 15
3.x = 30
x = 10
c) 2.(x - 11) = 200
x - 11 = 200 : 2
x - 11 = 100
x = 100 + 11
x = 111
d) 16 + 2.x = 30
2.x = 30 - 16
2.x = 14
x = 7
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1
a, (x + 17)- (24+35)
=x + 17 -24-35
=x+ ( -7) - 35
=x+(-42)
b, (-32) - ( y+ 20) +20
=-32-y-20+20
=-32-y + (-20) + 20
=-32-y + 0
=-32-y
Vậy.......
2.
a.(3567-214)-3567 b.(-2017)-(28-2017)
=3567-214-3567 =-2017-28+2017
=(3567-3567)-214 =(-2017+2017)-28
=0-214 = 0 -28
=-214 =-28
c.(269-357)+(269-357) d.(123+345)+(456-123)-[2017-(-345)]
=-269-357+269-357 =123+345+456-123-[2017+345]
=(-269+269)-(357+357) =0+345+456-2017-345
=0-714 =456-2017
=-714 =-1561
Học Tốt
# mui #
1. Đơn giản biểu thức:
a) ( x + 17) - ( 24 + 35)
x + 17 - 24 - 35
x + ( -42)
b) ( -32) - ( y + 20) + 20
- 32 - y - 20 +20
- 32 - y + 0
- y - 32
2. Tính nhanh các tổng sau:
a) ( 3567 - 214 ) - 3567
= 3567 - 214 - 3567
= ( 3567 - 3567 ) - 214
= 0 - 214
= -214
b) ( -2017) - ( 28 - 2017)
= -2017 - 28 + 2017
= ( 2017 - 2017 ) -28
= -28
c) -(269 - 357) + ( 269- 357)
= - 269 + 357 + 269 - 357
= ( 269 - 269 ) + ( 357 - 357)
= 0 + 0
= 0
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=\dfrac{1}{2}\left(\dfrac{2}{2\cdot3\cdot4}+\dfrac{2}{3\cdot4\cdot5}+...+\dfrac{2}{18\cdot19\cdot20}\right)\)
\(=\dfrac{1}{2}\left(\dfrac{1}{2\cdot3}-\dfrac{1}{3\cdot4}+\dfrac{1}{3\cdot4}-\dfrac{1}{4\cdot5}+...+\dfrac{1}{18\cdot19}-\dfrac{1}{19\cdot20}\right)\)
\(=\dfrac{1}{2}\left(\dfrac{1}{6}-\dfrac{1}{380}\right)\)
\(=\dfrac{187}{2280}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a)\left(3567-214\right)-3557\)
\(=3567-214-3567\)
\(=\left(3567-3567\right)-214\)
\(=0-214=-214\)
\(b)\left(-2017\right)-\left(28-2017\right)\)
\(=\left(-2017\right)-28+2017\)
\(=\left[\left(-2017\right)+2017\right]-28\)
\(=0-28=-28\)
\(c)-\left(269-357\right)+\left(269-357\right)\)
\(=\left(-269\right)+357+269-357\)
\(=\left[\left(-269\right)+269\right]+\left(357-357\right)\)
\(=0+0=0\)
\(d)\left(123+345\right)+\left(456-123\right)+\left[2017-\left(345\right)\right]\)
\(=123+345+456-123+2017-345\)
\(=\left(123-123\right)+\left(345-345\right)+456+2017\)
\(=0+0+456+2017\)
\(=2473\)
#Mạt Mạt#
\(2D=\dfrac{4-2}{2.3.4}+\dfrac{5-3}{3.4.5}+\dfrac{6-4}{4.5.6}+...+\dfrac{92-90}{90.91.92}=\)
\(=\dfrac{1}{2.3}-\dfrac{1}{3.4}+\dfrac{1}{3.4}-\dfrac{1}{4.5}+\dfrac{1}{4.5}-\dfrac{1}{5.6}+...+\dfrac{1}{30.91}-\dfrac{1}{91.92}=\)
\(=\dfrac{1}{2.3}-\dfrac{1}{91.92}\Rightarrow D=\left(\dfrac{1}{2.3}-\dfrac{1}{91.92}\right):2\)