Points P(1, 5) and Q(4, 1) on a coordinate grid represent side PQ of triangle PQR. What is the length of side PQ of the triangle?
Use your trusty Pythagorean Theorem. The line from P to Q is the diagonal of a right triangle with legs parallel to the axes.
PQ = sqrt((4-1)^2 + (1-5)^2)
= sqrt(9+16)
= 5