Mathematics crosswords

4. A type of number that can be expressed as the ratio of two integers.
7. A point on a line segment that divides the segment into two congruent segments.
9. The sum of the lengths of the sides of a polygon.
12. The steepness of a line expressed as a ratio, using any two points on the line.
13. The number which, when multiplied times a particular fraction, gives a result of 1.
16. A type of natural number that is divisible by 2.
17. A type of numbers used to count.
18. The point on an angle where the two sides intersect.

1. The middle number in a data set when the data are put in order.
2. Points that lie on the same line.
3. The ten symbols, 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.
5. A positive angle measuring less than 90 degrees.
6. The value of a variable that makes an equation true.
8. The top part of a fraction.
10. A parallelogram with four equal sides.
11. In a data set, the sum of all the data points, divided by the number of data points; average.
14. The union of two rays with a common endpoint.
15. The distance from the center to a point on a circle.