How do you find the first term of the arithmetic sequence having the data given #S_n= 781#, d = 3, n = 22?

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Answer 1 · 2016-03-21T20:56:25

Use the formula #s_n = n/2(2a + (n - 1)d)#

a is the first term.

#781 = 22/2(2a + (22 - 1)3)#

#781 = 11(2a + 63)#

#781 = 22a + 693#

#88 = 22a#

#4 = a#

Practice exercises:

An arithetic sequence of 14 terms has a sum of -322. The first term is -2. Find the common ratio, r.

Good luck.