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Solutions
Given:
\(P(n): 1 \times 3+2 \times 3^{2}+3 \times 3^{3}+\ldots+n \times 3^{n} \) \(=\frac{(2 n-1) 3^{n+1}+3}{4} \)
For \( n=1,\)
\(\text {L H.S }=1 \times 3=3\) and R.H.S. \(=\frac{(2 \times 1-1) 3^{1+1}+3}{4}=\frac{3^{2}+3}{4}=\frac{12}{4}=3\)
Thus \(\mathrm{P}(1)\) is true.
\(P(n)\) be true for some \(n=k\).
\(1 \times 3+2 \times 3^{2}+3 \times 3^{3}+\ldots+k \times 3^{k}\) \(=\frac{(2 k-1) 3^{k+1}+3}{4}\)
Now, \(P(n)\) is true for \(n=k+1\).
\(1 \times 3+2 \times 3^{2}+3 \times 3^{3}+\ldots+(k+1) \times 3^{k+1}\) \(=\frac{(2 k+1) 3^{k+2}+3}{4}\)
Adding \((k+1) \times 3^{k+1}\) on both sides, we get
\(1 \times 3+2 \times 3^{2}+3 \times 3^{3}+\ldots+k \times 3^{k}+(k+1) \times 3^{k+1} \) \(=\frac{(2 k-1) 3^{k+1}+3}{4}+(k+1) \times 3^{k+1} \)
\(=\frac{(2 k-1) 3^{k+1}+3 \mid 4(k+1) 3^{k+1}}{4} \)
\(=\frac{3^{k+1}[2 k-1+4(k+1)]+3}{4} \)
\(=\frac{3^{k+1}(6 k+3)+3}{4} \)
\(=\frac{3^{(k+1)+1}(2 k+1)+3}{4} \)
\(=\frac{(2 k+1) 3^{k+2}+3}{4}\)
Thus, \(P(k+1)\) is true whenever \(P(k)\) is true.
By the principle of mathematical induction, statement P(n) is true for all natural numbers.