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a) \(\left(x+1\right)+\left(x+4\right)+\left(x+7\right)+...+\left(x+28\right)=155\)
\(\Rightarrow10\times x+\left(1+4+7+...+28\right)=155\)
\(\Rightarrow10\times x+\frac{\left(1+28\right)\times10}{2}=155\)
\(\Rightarrow10\times x+145=155\)
\(\Rightarrow10\times x=10\Rightarrow x=1\)
b)

\(2^x+4.2^x=5.2^5\)
\(2^x.\left(1+4\right)=5.2^5\)
\(2^x.5=5.2^5\)
\(\Rightarrow2^x=2^5\) ( cùng chia hai vế cho 5)
\(\Rightarrow x=5\)
Vậy \(x=5\)
\(2^x+4.2^x=5.2^5\)
\(\Rightarrow2^x.\left(1+4\right)=5.2^5\)
\(\Rightarrow2^x.5=5.2^5\)
\(\Rightarrow x=5\)

\(a\)) \(x+0,5=\frac{3}{4}\)
\(x+\frac{5}{10}=\frac{3}{4}\)
\(x=\frac{3}{4}-\frac{5}{10}\)
\(x=\frac{1}{4}\)
b) \(3\frac{4}{5}-x=\frac{4}{5}\)
\(\frac{19}{5}-x=\frac{4}{5}\)
\(x=\frac{19}{5}-\frac{4}{5}\)
\(x=\frac{15}{5}=3\)
c) \(3\times x+2\times x=20\)
\(\left(3+2\right)\times x=20\)
\(5\times x=20\)
\(x=20\div5\)
\(x=4\)
d) \(2\div x=\frac{3}{4}\)
\(x=2\div\frac{3}{4}\)\(x=\frac{8}{3}\)


a, \(S_1=\left[\left(999-1\right):1+1\right].\frac{\left(1+999\right)}{2}\)\(=499500\)
d, \(S_4=\left[\left(126-24\right):1+1\right].\frac{24+126}{2}\)\(=7725\)
e, \(S_5=\left[\left(79-1\right):3+1\right].\frac{79+1}{2}\)\(=3160\)
X x 7 + X x 2 + X = 5/7
X x ( 7+2+1)=5/7
X x10=5/7
X = 5/7 :10
X=1/14
chúc làm bài tốt