Erwin Harahap

do Now or Never

suku ke-20 dari Sn=n^2+5n-14

Posted by Erwin on September 8, 2015

U_n = S_n-S_{n-1}
U_n = (n^2 + 5n - 14)-((n-1)^2 + 5(n-1) - 14)
U_n = (n^2 + 5n - 14)-(n^2 - 2n + 1 + 5n - 5 - 14)
U_n = n^2+5n-14-n^2+2n-1-5n+5+14
U_n = \not{n^2} + \not{5n} - \not{14}-\not{n^2}+2n-1-\not{5n}+5+\not{14}
U_n = 2n + 5 [/tex]latex

dengan demikian diketahui suku ke-n:
selanjutnya masukkan n = 20

U_{20} = 2(20) + 5
U_{20} = 45

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