Question

Find the algebraic expression to compute the third number of an arithmetic sequence, using the first and the second numbers.

Answer 1

Let the first, second and third number in the arithmetic sequence be x, y and z respectively.

Thus, the arithmetic sequence is x, y, z, …

⇒ y − x = z − y

⇒ z = y − x + y

⇒ z = 2y − x

Therefore, the algebraic expression to compute the third number of the arithmetic sequence using the first and the second numbers is z = 2y − x, where x, y and z are respectively the first, second and third numbers of the arithmetic sequence.