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=> 3A = 1.2. (3-0) +2.3.(4-1) + ...+18.19.(20-17)
=1.2.3-0.1.2+2.3.4-1.2.3+...+18.19.20-17.18.19
=(1.2.3-1.2.3)+(2.3.4-2.3.4)+...+(17.18.19-17.18.19)+18.19.20-0.1.2
=0+0+0+...+0+18.19.20
=18.19.20
=> A = 6.19.20= 114* 20= 2280
`A= 1.2 + 2.3 +3.4 +...+ 18.19`
`3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) +....+ 18.19.(20-17)`
`3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ....+ 18.19.20 - 17.18.19 `
`3A = 18.19.20`
`A = 6.19.20`
`A = 2280`
\(\text{a) }\left(27+514\right)-\left(486-73\right)\)
\(=27+514-486+73\)
\(=\left(27+73\right)+\left(514-486\right)\)
\(=100+1000\)
\(=1100\)
\(\text{b) }\left(38-29+43\right)-\left(43+28\right)\)
\(=38-29+43-43-28\)
\(=\left(38-28\right)+\left(43-43\right)-29\)
\(=10+0-29\)
\(=10-29\)
\(=-19\)
\(\text{c) }\left(-245-12\right)-\left(18-245\right)\)
\(=-245-12-18+245\)
\(=\left(245-245\right)-\left(12+18\right)\)
\(=0-30\)
\(=-30\)
\(\text{d) }\left(327+414\right)-\left(-586-73\right)\)
\(=327+414+586+73\)
\(=\left(327+73\right)+\left(414+586\right)\)
\(=400+1000\)
\(=1400\)
|2x-5|= 3
\(\Rightarrow\orbr{\begin{cases}2x-5=3\\2x-5=-3\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}2x=8\Rightarrow x=4\\2x=2\Rightarrow x=1\end{cases}}\)
Vậy x=4 hoặc =1
-125-(3x+1)=(-2)×(-77)
-125-(3x+1)=154
3x+1=-125-154
3x+1=-279
3x=-279-1
3x=-280
x=\(\frac{-280}{3}\)
Vậy...........................