WBJEE 2015 Math Solved Paper

Put n = 1, a + b = c + d ...... (1)
Put n = 3 a3 + b3 = c3 + d3 ............. (2)
from (1) and (2) ab = cd
Consider a quadratic with roots (a3,b3)
x2 – (a3 + b3)x + a3b3 = 0 ........ (3)
Consider another quadratic with roots (c3, d3)
x2 – (c3 + d3)x + (cd)3 = 0 ................. (4)
Since a3 + b3 = c3 + d3 and (cd)3 = (ab)3
Both quadratic are same and quadratic cannot have more than 2 roots.
Here a = c and b = d or a = d, b = c