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°
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°` 