Question 1177057
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3x^50 - 3 = 3*(x^50-1) = 3*((x^2)^25-1) = 3*(x^2-1)*(1 + x^2 + x^4 + x^6 + . . . + x^48).


Here I used the formula for the sum of an geometric progression


    1 + z + z^2 + z^3 + . . . + z^(n-1) = {{{(z^n-1)/(z-1)}}}


with z = x^2.
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Solved.