\(x+2x+3x+...+9x=459-3^2\)
\(\Rightarrow9x+\left(1+2+3+...+9\right)=450\)
\(\Rightarrow9x+\frac{\left[\left(9+1\right).9\right]}{2}=450\)
\(\Rightarrow9x+45=450\)
\(\Rightarrow9x=450-45\)
\(\Rightarrow x=\frac{450-45}{9}=\frac{405}{9}=45\)
\(2^{x+3}+2^x=144\)
\(\Rightarrow2^x\left(2^3+1\right)=144\)
\(\Rightarrow2^x.9=144\)
\(\Rightarrow2^x=\frac{144}{9}=16=2^4\)
Vậy \(x=4\)
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