All solutions here are SUGGESTED. Mr. Teng will hold no liability for any errors. Comments are entirely personal opinions.

(i)
$U_n = S_n - S_{n-1}$

$= n(2n+c) - (n-1)[2(n-1)+c]$

$= 4n-2+c$

(ii)
$U_{n-1} = 4(n+1)-2+c$

$U_{n+1} = U_n + 4$

Formulas!!! Every simple questions involving formulas!

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