MCQ
${\left( {x - \frac{1}{{2x}}} \right)^8}$ ના વિસ્તરણમાં ${x^2}$ નો સહગુણક મેળવો.
- A$\frac{1}{7}$
- B$\frac{{ - 1}}{7}$
- ✓$-7$
- D$7$
${ = ^8}{C_r}{\left( {\frac{{ - 1}}{2}} \right)^r}{x^{8 - r - r}}$
For coefficient of ${x^2}$, $8 - 2r = 2$
==> $r = 3$
Coefficient of ${x^2}{ = ^8}{C_3}{\left( {\frac{{ - 1}}{2}} \right)^3}$= $ - \frac{{8!}}{{5!\,3!}}\frac{1}{{{2^3}}} = - 7$.
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