So sánh \(\dfrac{1}{11}\)+ \(\dfrac{1}{12}\) +...+ \(\dfrac{1}{19}\)+ \(\dfrac{1}{20}\) với \(\dfrac{2}{3}\)
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\(a)A\text{=}-5^{22}+22+112-100+5^{22}+2022\text{=}\left(-5^{22}+5^{22}\right)+\left(22+112-100+2022\right)\text{=}0+2056\text{=}2056\)
\(b)B\text{=}\dfrac{2^{10}.13+2^{10}.65}{2^8.104}\text{=}\dfrac{2^{10}.\left(13+65\right)}{2^{10}.26}\text{=}\dfrac{78}{26}\text{=}3\)
Ta có : \(A\text{=}\dfrac{2013.2014-1}{2013.2014}\text{=}\dfrac{2013.2014}{2013.2014}-\dfrac{1}{2013.2014}\text{=}1-\dfrac{1}{2013.2014}\)
\(B\text{=}\dfrac{2014.2015-1}{2014.2015}\text{=}\dfrac{2014.2015}{2014.2015}-\dfrac{1}{2014.2015}\text{=}1-\dfrac{1}{2014.2015}\)
\(Ta\) có : \(\dfrac{1}{2013.2014}>\dfrac{1}{2014.2015}\)
\(\Rightarrow A< B\)
\(\dfrac{7}{8}+\dfrac{7}{8}:\dfrac{1}{8}-\dfrac{1}{2}\\ =\dfrac{7}{8}+\dfrac{7}{8}\cdot\dfrac{8}{1}-\dfrac{1}{2}\\ =\dfrac{7}{8}+\dfrac{7}{1}-\dfrac{1}{2}\\ =\dfrac{7}{8}+\dfrac{56}{8}-\dfrac{4}{8}\\ =\dfrac{59}{8}\)
\(\left(x-3\right)^6=\left(x-1\right)^2\)
\(\Rightarrow\left(x-3\right)^{3^2}=\left(x-1\right)^2\)
\(\Rightarrow\left(x-3\right)^3=x-1\)
a)
ta có 2n-1=2(n-3)+7
2(n-3)⋮n-3;để 2n-1⋮n-3
<=>7⋮n-3
=>n-3ϵƯ(7)={1;-1;7;-7}
lập bảng
n-3 | 1 | -1 | 7 | -7 |
n | 4 | 2 | 10 |
-4 |
vậy nϵ{4;2;10;-4}
a)
\(\dfrac{6}{13}\cdot\dfrac{8}{7}\cdot\dfrac{-26}{3}\cdot\dfrac{-7}{8}\)
\(=\dfrac{6}{13}\cdot\dfrac{-26}{3}\cdot\dfrac{8}{7}\cdot\dfrac{-7}{8}\)
\(=-4\cdot\left(-1\right)\\ =4\)
b)
\(\dfrac{6}{11}+\dfrac{11}{3}\cdot\dfrac{3}{22}\)
\(=\dfrac{6}{11}+\dfrac{1}{2}\\ =\dfrac{12}{22}+\dfrac{11}{22}\\ =\dfrac{23}{22}\)
Lập bảng :
x -5 -1 2
x + 5 - 0 + | + | +
x - 2 - | - | - 0 +
x + 1 - | - | + | +
Từ bảng xét dấu ta có:
với x < -5 ta có: -( x + 5) - 3.[ - (x-2) ] = -2(x+1)
-x - 5 + 3x + 6 = -2x - 2
2x + 2x + 1 + 2 = 0
4x = -3
x = - 3/5 (loại)
Với -5 ≤ x ≤ 1 ta có : -( x + 5) + 3( x -2) = 2(x +1)
-x - 5 + 3x - 6 = 2x + 2
2x - 11 = 2x + 2 ( vô lý loại )
Với -1 < x < 2 ta có : (x+5) + 3(x-2) = 2(x+1)
x + 5 + 3x - 6 = 2x + 2
4x - 1 - 2x - 2 = 0
2x - 3 = 0
x = 3/2 (nhận)
Với 2 ≤ x ta có: (x+5) - 3(x-2) = 2( x+1)
x + 5 -3x + 6 = 2x + 2
2x - x + 3x = 5+6 - 2
4x = 9
x = 9/4 (nhận )
Kết luận x ϵ { 3/2; 9/4}
\(\dfrac{2323}{1818}\) . \(\dfrac{7272}{6969}\)
= \(\dfrac{23\cdot101}{18\cdot101}\) . \(\dfrac{18\cdot4\cdot101}{23\cdot3\cdot101}\)
= \(\dfrac{23}{18}\cdot\dfrac{18\cdot4}{23\cdot3}=\dfrac{4}{3}\)