12. A number cannot be irrational and an integer.
True. An integer is a whole number that can be positive, negative, or zero (e.g., -2, -1, 0, 1, 2). An irrational number, on the other hand, is a number that cannot be expressed as a fraction of two integers, meaning it cannot be written in the form \( \frac{a}{b} \) where \( a \) and \( b \) are integers and \( b \neq 0 \). Since all integers can be expressed as a fraction (for example, the integer 2 can be expressed as \( \frac{2}{1} \)), they are rational. Therefore, a number cannot be both irrational and an integer.
13. All integers are rational.
True. By definition, a rational number is any number that can be expressed as the quotient of two integers, where the denominator is not zero (i.e., in the form \( \frac{a}{b} \) with \( b \neq 0 \)). All integers can be written as a fraction where the denominator is 1 (e.g., the integer 3 can be expressed as \( \frac{3}{1} \)). Hence, all integers are indeed rational numbers.