Question
The 5th and 8th term of an arithmetic progression are 13
and -5 respectively. What is the 15th term?Solution
Given: 5th term a5 = 13, 8th term a8 = −5.
Use the formula: The formula for the n-th term is: 
Solve the system of equations: Subtract the first equation from the second: 3d = −18 ⇒ d = − 6 Substitute d = − 6 into a1+ 4d = 13: 
Find the 15th term:
The 15th term is -47.
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