I got this question wrong on an ACT practice test, but it offered no explanation, just the answer.
58. What is the sum of the first four terms of the arithmetic sequence in which the sixth term is 8 and the tenth term is 13?
. . . . .F. 10.5
. . . . .G. 14.5
. . . . .H. 18
. . . . .J. 21.25
. . . . .K. 39.5
I was just wondering if some of you could explain how to solve it the fastest and most efficient way. I would have used the formula S=n(a1+an)/2, but it does not give the first term. Thanks for the help!
58. What is the sum of the first four terms of the arithmetic sequence in which the sixth term is 8 and the tenth term is 13?
. . . . .F. 10.5
. . . . .G. 14.5
. . . . .H. 18
. . . . .J. 21.25
. . . . .K. 39.5
I was just wondering if some of you could explain how to solve it the fastest and most efficient way. I would have used the formula S=n(a1+an)/2, but it does not give the first term. Thanks for the help!
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