help and explain to me how to solve it ?

[kingboy]

1)in the binomial expansion of (1+1/3 x)^n , the coefficients of the fourth and fifth term are equal. Find the value of n.
2)the coefficient of x^5 in binomial expansion of (1+5x)^8 id the same as the coefficient of x^4 in the expansion of (a+5x)^7.find the value of a.
3)if the first three terms in the expansion of (1+ax)^n in ascending powers of x are 1-4x+7x^2, find n and a.

 

[Deleted member 4993]

1)in the binomial expansion of (1+1/3 x)^n , the coefficients of the fourth and fifth term are equal. Find the value of n.
2)the coefficient of x^5 in binomial expansion of (1+5x)^8 id the same as the coefficient of x^4 in the expansion of (a+5x)^7.find the value of a.
3)if the first three terms in the expansion of (1+ax)^n in ascending powers of x are 1-4x+7x^2, find n and a.

Hint: What is the n[sup:2fvoqatg]th[/sup:2fvoqatg] term of (a + b)[sup:2fvoqatg]k[/sup:2fvoqatg] where k>n ?