If the number of terms in the expansion of (1− 2 x
+ 4 x2) n
,x ≠ 0, is 28, then the sum of the
coefficients off all the terms in this expansion is:-

This ques is wrong as per JEE .. Here`s the proof.There are three terms in the expansion. Let`s look at the maximum (or minimum) possible power of x in the expansion. 1^(n) will be x^0th term (highest power), (1/x)^(n) will be x^(-n)th term and (1/(x^2))^n will be the x^(-2n)th term (lowest power). Every single power of x between the highest (0) and the lowest (-2n) will be present in the expansion. This implies that -Total number of terms in the expansion would be 2 n+1 and in the question it is given that it is 28.Which gives n=13.5 . Which is not possible.