SOLUTION: Given a1=3 and a5=17 (the first and fifth number numbers in an arithmetic sequence), what is the 20th term?

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Question 1197589: Given a1=3 and a5=17 (the first and fifth number numbers in an arithmetic sequence), what is the 20th term?
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

Given:
+a%5B1%5D=3
+a%5B5%5D=17

an arithmetic sequence formula is:
+a%5Bn%5D=a%5B1%5D+%2Bd%28n-1%29
find common difference d
+a%5B5%5D=3%2Bd%285-1%29
+a%5B5%5D=3%2B4d............since given +a%5B5%5D=17
+3%2B4d=17
+4d=17-3
+4d=14
+d=14%2F4
+d=3.5
your formula is:
+a%5Bn%5D=3+%2B3.5%28n-1%29

then, the 20th term will be:
+a%5B20%5D=3+%2B3.5%2820-1%29
+a%5B20%5D=3+%2B3.5%2819%29
+a%5B20%5D=69.5