А₁ = 10, a, 19.

It appears that you have provided the first and third terms of an arithmetic sequence (or arithmetic progression), where:

a₁ = 10 (the first term is 10) a₃ = 19 (the third term is 19)

The common difference (d) in an arithmetic sequence can be calculated using two terms of the sequence. If 'a' represents the second term (which you have indicated with a placeholder), we can express the third term (a₃) as follows:

a₃ = a₁ + 2d

Substituting the given terms:

19 = 10 + 2d

Solving for d:

2d = 19 - 10 2d = 9 d = 9 / 2 d = 4.5

Now, to find the missing second term (a), we can use the fact that each term in the sequence is the previous term plus the common difference:

a₂ = a₁ + d

Plugging in the values we have:

a₂ = 10 + 4.5 a₂ = 14.5

So, the first three terms of the arithmetic sequence are 10, 14.5, and 19.