Chapter 4 - Exponential and Logarithmic Functions - Section 4.2 One-to-One Functions; Inverse Functions - 4.2 Assess Your Understanding - Page 290: 5

$f(x_1) \ne f(x_2)$

The definition of a one-to-one function is a function that a horizontal line will intersect at no more than one point. Therefore, the function can be one-to-one, when the different inputs attain different outputs. That is, $f(x_1) \ne f(x_2)$. The function values must be distinct.