`A=1+x+x^2+x^3 +....+x^49`
=>`Ax =x+x^2+x^3 +....+x^49 + x^50`
=>`Ax - A = (x+x^2+x^3 +....+x^49 + x^50) - (1+x+x^2+x^3 +....+x^49)`
=>`Ax - A = x+x^2+x^3 +....+x^49 + x^50 - 1-x-x^2-x^3-...-x^49`
=>`Ax - A = (x-x)+(x^2 - x^2)+(x^3-x^3)+...+(x^49-x^49) + x^50 - 1`
=>`Ax - A = 0 + 0 + 0 +...+ 0 + x^50 - 1`
=>`Ax - A = x^50 - 1` (đpcm)