To find the sum of the interior, first you find the number of sides the polygon has.
A decagon has 10 sides, so let's have "n" represent the sides of our polygon.
We then take n and subtract 2, so now we end up with 8. Now we multiply by 180 and we end up with 1,440. The is the sum of the interior angles of a decagon. Now we use the (n-2) x 180 formula with the triangle.
Triangles have 3 sides, and minus 2 equal 1. 1 x 180 is 180, so we do 1,440 - 180 and we end up with 1,260.
Your answer is 1,260
