STP is a right, isosceles triangle.
The diagonals of a square cut the square itself in two equal right isosceles triangles. So, both diagonals cut the square in four right isosceles triangles.
STP is a right triangle because the two diagonal are perpendicular to each other, and it is isosceles because ST and TP are both half a diagonal, because the two diagonals intersect in their midpoint.
So, the angles of STP are 90, 45 and 45 degrees.
As for the lengths, SP is a side of the square, so let's call its length [tex] l [/tex]. The diagonals of a square are [tex] l\sqrt{2} [/tex] units long, and so [tex] ST = TP = \frac{l\sqrt{2}}{2} [/tex]
