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7.Binomial Theorem
medium
$(x+2 y)^{9}$ के प्रसार में $x^{6} y^{3}$ का गुणांक ज्ञात कीजिए।
A
$672$
B
$672$
C
$672$
D
$672$
Solution
Suppose $x^{6} y^{3}$ occurs in the $(r+1)^{\text {th }}$ term of the expansion $(x+2 y)^{9}$
Now ${T_{r + 1}} = {\,^9}{C_r}{x^{9 – r}}{(2y)^r} = {\,^9}{C_r}{2^r} \cdot {x^{9 – r}} \cdot {y^r}$
Comparing the indices of $x$ as well as $y$ in $x^{6} y^{3}$ and in $T_{r+1},$ we get $r=3$
Thus, the coefficient of $x^{6} y^{3}$ is
${\,^9}{C_3}{2^3} = \frac{{9!}}{{3!6!}} \cdot {2^3} = \frac{{9.8.7}}{{3.2}} \cdot {2^3} = 672$
Standard 11
Mathematics
