Across
1
Lines that never intersect
5
Repeated addition
6
Value that does not change
10
Part of a circle's circumference
11
Reference line in a graph
Down
2
Symbol representing unknown value
3
Rectangular array of numbers
4
The number representing nothing
7
Sum divided by count
8
Three-sided polygon
9
Part of a line with two endpoints
12
Number divisible only by 1 and itself
13
Collection of distinct elements