The total interior angles of a heptagon = 900°

Each time we add a side (triangle to square, square to pentagon, pentagon to hexagon) we add another 180°.

Triangle to heptagon

Triangle

Square

Pentagon

Hexagon

Heptagon

$180°$

$+$

$180°$

$+$

$180°$

$+$

$180°$

$+$

$180°$

$=$

$900°$

The total internal angles of a heptagon = 900°

The interior angles of a heptagon equal 900 degrees

Now divide the total internal angle by the number of corners.

i.e. divide 900° by 7.

$900^circ/7=128.57^circ$

1

2

3

4

5

6

7

$128.57°$

$+$

$128.57°$

$+$

$128.57°$

$+$

$128.57°$

$+$

$128.57°$

$+$

$128.57°$

$+$

$128.57°$

$=$

$900°$