Across
1
Middle value in an ordered dataset
4
Total distance around a shape
5
Exact correspondence on both sides
10
Single numeral from 0 to 9
11
Lines that never intersect
13
Line crossing from corner to corner
14
Three-sided polygon
Down
2
Collection of distinct elements
3
Repeated addition
6
Line touching circle at one point
7
Number less than zero
8
The number representing nothing
9
Symbol representing unknown value
12
Steepness of a line