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\(A=\frac{\left[\left(25-1\right):1+1\right]\left(25+1\right)}{2}=325.\)
\(B=\frac{\left[\left(51-3\right):2+1\right]\left(51+3\right)}{2}=675\)
\(C=\frac{\left[\left(81-1\right):4+1\right]\left(81+1\right)}{2}=861\)
\(\left(x+15\right):3=7\)
\(x+15=7.3\)
\(x+15=21\)
\(x=21-15\)
\(x=6\)
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\(4.\left(6-x\right)=280:36\)
\(4.\left(6-x\right)=\dfrac{70}{9}\)
\(6-x=\dfrac{70}{9}:4\)
\(6-x=\dfrac{35}{18}\)
\(x=6-\dfrac{35}{18}\)
\(x=\dfrac{73}{18}\)
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\(5x-3x-12=8\)
\(2x=8+12\)
\(2x=20\)
\(x=\dfrac{20}{2}\)
\(x=10\)
\(\left(x+15\right):3=7\\ \Rightarrow x+15=7.3=21\\ \Rightarrow x=21-15=6\)
\(4\left(6-x\right)=280:36\\ \Rightarrow4\left(6-x\right)=\dfrac{70}{9}\\ \Rightarrow6-x=\dfrac{70}{9}:4\\ \Rightarrow6-x=\dfrac{35}{18}\\ \Rightarrow x=6-\dfrac{35}{18}=\dfrac{73}{18}\)
\(5x-3x-12=8\\ \Rightarrow2x=8+12=20\\ \Rightarrow x=\dfrac{20}{2}=10\)
a) \(3\left(x-5\right)+6=36\)
\(\Rightarrow3\cdot\left(x-5\right)=36-6\)
\(\Rightarrow3\cdot\left(x-5\right)=30\)
\(\Rightarrow x-5=10\)
\(\Rightarrow x=10+5\)
\(\Rightarrow x=15\)
b) \(200-8\cdot\left(x+7\right)=24\)
\(\Rightarrow8\cdot\left(x+7\right)=200-24\)
\(\Rightarrow8\cdot\left(x+7\right)=176\)
\(\Rightarrow x+7=\dfrac{176}{8}\)
\(\Rightarrow x+7=22\)
\(\Rightarrow x=22-7\)
\(\Rightarrow x=15\)
c) \(\left(x-890\right)\left(74-x\right)=0\)
+) \(x-890=0\)
\(\Rightarrow x=890\)
+) \(74-x=0\)
\(\Rightarrow x=74\)
d) \(\left(31-x\right)\cdot\left(x-64\right)=0\)
+) \(31-x=0\)
\(\Rightarrow x=31\)
+) \(x-64=0\)
\(\Rightarrow x=64\)
Bài 3:
1: =>2x=-5/3-1/2=-10/6-3/6=-13/6
hay x=-13/12
2: =>3/5x=1/7+3/5=5/35+21/35=26/35
hay x=26/3
3: =>-3x=5/6+3/4=10/12+9/12=19/12
hay x=-19/36
4: =>1/2x=3/7-5/4=12/28-35/28=-23/28
hay x=-23/14
5: =>1/4x=-3/5-7/5=-2
hay x=-8
6: =>3x=1/42+1/7=1/42+6/42=1/7
hay x=1/21
\(6\left(x\cdot3\right)\)
\(=6\cdot3x\)
\(=18x\)