# The first term of arithmetic progression is a1=1 and the fourth term is 7. What is the common difference?

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### 1 Answer

We'll recall what is the formula that gives any term of a arthmetic progression:

an = a1 + (n-1)*d

an is the n-th term

a1 is the 1st term

d is the common difference

Since we know the 4th and the 1st terms, we could write:

a4 = a1 + (4-1)*d

7 = 1 + 3d

We'll use the symmetrical property:

3d = 7 - 1

3d = 6

d = 2

**The requested common difference of the given arithmetic progression is d = 2.**