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Sum of an arithmetic sequence, from two partial sums

ÀÛ¼ºÀÚ Uploader : prove ÀÛ¼ºÀÏ Upload Date: 2018-08-12º¯°æÀÏ Update Date: 2018-08-13Á¶È¸¼ö View : 329

For an arithmetic sequence, with the first term and the common
difference being a and d, the sum of sequence up to nth term is :
Sn = a + d(n-1)

Letting other sums up to n1th and n2th term being Sn1 = N1 and Sn2 = N2, Sn is expressed as:

Sn1 = a + d(n1-1) = N1
Sn2 = a + d(n2-1) = N2
Sn1-Sn2 = d(n1-n2) = N1-N2
d = (N1-N2)/(n1-n2)
a = N1 - d(n1-1)  
 = N1 - (n1-1)*(N1-N2)/(n1-n2)
Sn = N1 - d(n1-1) + d(n-1)
   = N1 - d(n1-n)  
   = N1 - (n1-n)*(N1-N2)/(n1-n2)

*** Âü°í¹®Çå[References] ***

Sn = N1 - (n1-n)*(N1-N2)/(n1-n2)
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