7+12+17+22+27+.+5n+2=

Original formula = 1 * 5 + 2 + 2 * 5 + 2 +... + n * 5 + 2
=5*(1+2+3+...+n)+2*n
=5*(1+n)*n/2+2n
=2.5n^2+2.5n+2n
=2.5n ^ 2 (n square) + 4.5n

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