posted by lindsay .
The 9th term of an AP is 8 and the 4th term is 20. Find the first term and the common difference.
Let d be the difference between terms.
Let x be the first term.
The second term will be x + d.
The third term will be x + 2d.
Fourth term will be x + 3d.
But the fourth term is 20, so x + 3d = 20.
Continue this until you get the eighth term.
Write an equation about the eighth term.
Now you have two equations with two unknowns which you can solve to find both the first term and the difference.