Or search by topic
A An octagon has $20$ diagonals
B A hexagon has $9$ diagonals
C A hexagon has $4$ more diagonals than a pentagon
D A pentagon has the same number of diagonals as it has sides
E A quadrilateral has twice as many diagonals as it has sides
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
An investigation involving adding and subtracting sets of consecutive numbers. Lots to find out, lots to explore.