The diagonal of a square connects the points (8,-2) and (-1,-11). Find the perimeter of the square.
The length of the diagonal is sqrt[(x2-x1)^2+(y2-y1)^2]
= sqrt [9^2 + 9^2] = 9 sqrt2
Since the diagonal of a square is sqrt2 times longer than a side, the side length must be 9.
Now calculate the perimeter.