This gives 8 × 180 = 1440. The interior angles in a decagon sum to 1440°. The second method is to use the formula: sum of interior angles = (𝒏 – 2) × 180. This gives (10 – 2) × 180 ...

But do you know how to insert a sum without typing the formula? Have you struggled to start a new line of text within a cell? Well, you need to learn these less popular but extremely useful shortcuts.

To check your work, click the cell with the sum and look at the formula bar atop the screen. It will show the details of the calculation. Insider receives a commission when you buy through our links.

Add the known angles. Form an equation equal to 360˚. Subtract the sum of the known angles from the sum of a complete turn (360˚). Triangles have three interior angles. They are formed at the ...