SOLUTION: find the formula for a_n if a_1 = -4 and a_5 = 16. This is AP

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Question 1087018: find the formula for a_n if a_1 = -4 and a_5 = 16. This is AP
Answer by ikleyn(52778) About Me  (Show Source):
You can put this solution on YOUR website!
.
a%5B5%5D = a%5B1%5D%2B4%2Ad = -4 + 4*d = 16  ====>  4d = 20  ===>  d = 5.


The formula is a%5Bn%5D = -4 + 5*(n-1) = -4 +5n - 5 = -9 + 5n,  n = 1, 2, 3, . . . 


Answer. The general formula is  a%5Bn%5D = -4 + 5*(n-1)  or  (which is the same)  a%5Bn%5D =  -9 + 5n,  n = 1, 2, 3, . . .