Chapter 10 - Section 10.1 - The Rectangular Coordinate System - Exercises - Page 449: 8d

$d = 2 \sqrt {(c - a)^{2} + (d - b)^{2}}$

$d = \sqrt {(2c - 2a)^{2} + (2d - 2b)^{2}}$ $d = \sqrt {(2(c - a))^{2} + (2(d - b))^{2}}$ $d = \sqrt {4(c - a)^{2} + 4(d - b)^{2}}$ $d = \sqrt {4((c - a)^{2} + (d - b)^{2})}$ $d = 2 \sqrt {(c - a)^{2} + (d - b)^{2}}$