а₁ = 10, a, 19.

It seems that you might have intended to provide a sequence with a missing term, labeled "a", and are asking me to identify that term. The sequence you've given is:

a₁ = 10, a₂ = a, a₃ = 19.

To determine the missing term (a₂ = a), we need more information about the pattern or rule that the sequence follows. Sequences can follow arithmetic rules, geometric rules, or even more complex formulas, and without this information, it would be impossible to accurately determine what the missing term is.

Here are examples of how we could find the missing term with additional information:

1. If the sequence is arithmetic (each term is a constant difference from the previous term), then we can find that difference and use it to calculate the missing term. For instance, if a₃ - a₂ = a₂ - a₁, then: 19 - a = a - 10 2a = 29 a = 29 / 2 a = 14.5

2. If the sequence is geometric (each term is a constant ratio to the previous term), then we can find that ratio and use it to calculate the missing term. For instance, if a₃ / a₂ = a₂ / a₁, then: 19 / a = a / 10 a² = 190 a = sqrt(190) a ≈ 13.78

Please provide additional information about the sequence if you would like a more specific answer.