Posted by Brittany on Wednesday, February 2, 2011 at 3:52pm.
Arithmetic sequence
a, a + d, a + 2d, a + 3d, ...a +(n - 1)d
1)a3 = 10, a7 = 20
Since these are 4 places apart (7 - 3 = 4)
a7 = a3 + 4d
20 = 10 + 4d
10 = 4d
d = 10/4 = 2.5
So, the common difference, d = 2.5
To find the 1st term a,
a3 = a + 2d
10 = a + 2(2.5)
10 = a + 5
a = 5
2) to find 9th term
a9 = a + 8d
a9 = 5 + 8(2.5)
a9 = 5 + 20
a9 = 25
3) nth term
Last term = a + (n - 1)d
an = 5 + (n - 1)(2.5)
4a2
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