Find the term independent of x in the expansion of \(\left( x - \frac{1}{x^2} \right)^9\).
Solution
To find the term independent of x in the expansion of \(\left( x - \frac{1}{x^2} \right)^9\), we use the binomial theorem:
\(\left( x - \frac{1}{x^2} \right)^9 = \sum_{k=0}^{9} \binom{9}{k} x^{9-k} \left( -\frac{1}{x^2} \right)^k\)
The general term is:
\(T_k = \binom{9}{k} x^{9-k} \left( -\frac{1}{x^2} \right)^k = \binom{9}{k} (-1)^k x^{9-k-2k}\)
\(= \binom{9}{k} (-1)^k x^{9-3k}\)
We need the exponent of x to be zero for the term to be independent of x:
\(9 - 3k = 0\)
\(3k = 9\)
\(k = 3\)
Substitute \(k = 3\) into the general term:
\(T_3 = \binom{9}{3} (-1)^3 x^{9-3 \times 3}\)
\(= \binom{9}{3} (-1)^3 x^0\)
\(= \binom{9}{3} (-1)^3\)
\(= 84 \times (-1)\)
\(= -84\)
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