How do you find #S_n# for the arithmetic series given #a_1=7#, #a_n=79# and n=8?

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Answer 1 · 2017-09-14T16:42:29

#S_n = 344#

Given...

#S_n = ?#

#n = 8#

#a_1 = "First term" = 7#

#a_n = "Last term" = 79#

Recall #-> S_n = n/2 (a + L)#

#rArr S_n = n/2 (a + L)#

Substituting the values...

#rArr S_n = 8/2 (7 + 79)#

#rArr S_n = 8/2 (86)#

#rArr S_n = 4(86)#

#rArr S_n = 344#