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Galina-37 [17]
2 years ago
9

In the expansion of ( x^3 - 2/x^2 ) ^10 , find the coefficient of 1/x^5​

Mathematics
1 answer:
Lilit [14]2 years ago
3 0

Answer:

240

Step-by-step explanation:

We need to find the coeffeicent of the binomial expansion of

( {x}^{3}  - 2 {x}^{ - 2} ) {}^{10}

Note that

- 2 {x}^{ - 2}  = -  \frac{2}{ {x}^{2} }

The binomial theorem states that

(x + y) {}^{n}  = x {}^{n} y {}^{0}  +  \binom{n}{1} x {}^{n - 1} y +  \binom{n}{2} x {}^{n - 2} y {}^{2} ....... + x {}^{0} y {}^{n} ( \binom{n}{n} )

Using this, we let expand our series

( {x}^{3}   - 2 {x}^{ - 2} ) {}^{10}  = x {}^{30}  +  \binom{10}{1} ( {x}^{27}     2 {x}^{ - 2} ) +  \binom{10}{2}  {x}^{24} 2x {}^{ - 4}  +  \binom{10}{3}  {x}^{21} 2x {}^{ - 6}  +  \binom{10}{4}  {x}^{18} 2x { }^{ - 8}  +  \binom{10}{5} x {}^{15} 2x {}^{ - 10}  +  \binom{10}{6} x {}^{12}2 x {}^{ - 12}  +  \binom{10}{7} x {}^{ 9} 2x {}^{ - 14}  +  \binom{10}{8} x {}^{ 6} 2x {}^{ - 16}  +  \binom{10}{9} ( {x}^{3} )2x {}^{ - 18}  + 2x {}^{ - 20}

\frac{1}{ {x}^{5} }  = x {}^{ - 5}

So what term in the series eqaul x^-5.

That term is the 10 choose 7 term.

\binom{10}{7}  {x}^{9} 2x {}^{ - 14}

Because

=  \binom{10}{7} 2x {}^{ - 14}  {x}^{9}  =  \binom{10}{7} 2 {x}^{ - 5}

So we need to compute 10 choose 7.

That equals

10!/3!(7!)= 10×9×8/6= 720/6=120.

So we get

120(2) {x}^{ - 5}

240 {x}^{ - 5}

So the coeffceint u

is 240

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