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Let m be the smallest positive integer such that the coefficient of `x^2` in the expansion of `(1+x)^2 + (1 +x)^3 + (1 + x)^4 +........+ (1+x)^49 + (1 + mx)^50` is `(3n + 1) .^51C_3` for some positive integer n. Then the value of n is |
Answer» Coefficient of `x^(2)` in expansion `= 1+.^(3)C_(2)+.^(4)C_(2)+.^(5)C_(2) + "….." + .^(49)C_(2)+.^(50)C_(2).m^(2)` [as `.^(n)C_(r)+.^(n)C_(r-1) = .^(n+1)C_(r)`] `= (.^(3)C_(5)+.^(3)C_(2)) + .^(4)C_(2) + .^(5)C_(2) + "…." + .^(49)C_(2) + .^(50)C_(2)m^(2)` `= (.^(4)C_(3) + .^(4)C_(2)) + "....." + .^(50)C_(2)m^(2)` `= .^(5)C_(3) + .^(50)C_(2)m^(2) + .^(50)C_(2)m^(2)` `= .^(50)C_(3) + .^(50)C_(2)m^(2)+.^(50)C_(2)-.^(50)C_(2)` `= .^(51)C_(3)+.^(50)C_(2)(m^(2)-1)` `= (3n+1).^(51)C_(3)` (given) `:. 3n.(51)/(3).^(50)C_(2) = .^(50)C_(2)(m^(2) - 1)` `(m^(2)-1)/(51) = n` Value of n is 5. |
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