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