there must be a better answer ( 5 steps) to show you are smarter:), and I figured out.
1) draw one diagonal line and extend it long enough
2) select one of vertex on the diagonal line as center, and length of a side as radius to draw a circle to find two points on the diagonal line, one inside the square, one outside,
3) use the point ourside the square as the center, and length of the diagonal as radius to draw another circle, which will produce two points by intersecting the previous cirle.
4,5) link the point on diagonal (inside square) with the two points intersecting points on the circle,but not extend ourside the orignal square.
