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प्रश्न
Select the correct answer from the given alternatives.
If the coefficient of x2 and x3 in the expansion of (3 + ax)9 are the same, then the value of a is
विकल्प
`-7/9`
`-9/7`
`7/9`
`9/7`
उत्तर
`9/7`
Explanation;
tr+1= 9Cr(3)9–r(ax)r = 9Cr39–rarxr
∴ 9C2 × 37 × a2 = 9C3 × 36 × a3
∴ `(9!)/(2!7!) xx 3 = (9!)/(3!6!) xx "a"`
∴ a = `9/7`
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