7.Binomial Theorem
easy

${(a + b)^n}$ ના વિસ્તરણમાં ચોથાપદ નો સહગુણક 56 હોય, તો $n$ મેળવો.

A

$12$

B

$10$

C

$8$

D

$6$

Solution

(c) ${T_4} = {T_{3 + 1}}$

$ = \,{}^n{C_3}\,\,\,{a^{n – 3}}{b^3}$

==> ${}^n{C_3} = 56$

==> $\frac{{n!}}{{3\,!\,\,\,\left( {n – 3} \right)\,!}} = 56$

==> $n(n – 1)(n – 2) = 56.6$

==> $n(n – 1)(n – 2) = 8\,.\,7\,.\,6$

==> $n = 8$.

Standard 11
Mathematics

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