Final answer:
Given the equation x + y = 19 with the condition that x < 9, it must be true that y > 10 to ensure the sum equals 19.
Step-by-step explanation:
The equation given is x + y = 19 with the condition x < 9. To determine what must be true given these conditions, we'll use some basic algebra. Since x is less than 9, to achieve the sum of 19, y must compensate for the difference. Thus, y > 10 must be true because if x were 8 (the highest it could be while still being less than 9), y would have to be 11 to sum to 19 (8 + 11 = 19). If x is smaller, y must increase correspondingly.