Question 421448
Using the distance formula, {{{sqrt((x2 -x1)^2 + (y2 - y1)^2 )}}}
I determined the distance between the points.
st  = 2.8242
ut = 5.6568
su = 8.4852

By the triangle inequality thereom, the sum of any two sides of a triangle is > than the length of the third side.

adding st and ut, the result is < su. This would not fit the criteria for the vertexes of a triangle.

For the second set,

de = 6.4
ef = 4.0
fd = 5.4

This example does satisfy the triangle inequality thereom, it will be a valid triangle.

Cleomenius.