SOLUTION: the 13th and 18th terms of AP are 25 and 45 respctively. (a) calculate the common difference. (b) calculate its first term

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Question 38904This question is from textbook new general mathematics
: the 13th and 18th terms of AP are 25 and 45 respctively.
(a) calculate the common difference.
(b) calculate its first term This question is from textbook new general mathematics

Answer by fractalier(6550) About Me  (Show Source):
You can put this solution on YOUR website!
Okay, we have
a13 = 25 and a18 = 45
Thus we know there were five differences (18-13) that add to 45-25 = 20.
So the common difference is 4.
The general formula is
a-sub-n = a-sub-1 + (n - 1)d
a13 = 25 = a-sub-1 + (13 - 1)4
25 = a-sub-1 + 48
and
a-sub-1 = -23