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U => 7+7^2+7^3+7^4.....+7^84

Răspuns :

U(7+7²+7³+7⁴+....+7⁸⁴) =

= U(7+7²+7³+7⁴+....+7⁸¹+7⁸²+7⁸³+7⁸⁴)

= U[(7+9+3+1)+(7+9+3+1)+...+(7+9+3+1)]

{de 21 de ori, fiindcă 84/4 = 21 rest 0}

= U(20+20+...+20)

= 0