UV is a perpendicular bisector of RT. A perpendicular bisector UV is a line that does two things:
cuts the line segment RT into two equal pieces or bisects it;
makes a right angle with the line segment RT (is perpendicular).
Consider an arbitrary point X on the line RT that differs from S. Connect points U, V and X to form triangle UVX. Line XS will be the height and the median of the triangle UVX. This gives you that triangle UVX will be isosceles triangle with base UV.
It will be an isosceles triangle