Update appendix-e.md

Author: pavly-gerges

calculus/appendix-e.md

@@ -14,7 +14,7 @@ $$||v|| = \sqrt{{v_x}^2 + {v_y}^2 + {v_z}^2}$$
 
 $$||u|| = \sqrt{{u_x}^2 + {u_y}^2 + {u_z}^2}$$
 
-$$||w|| = ||u-v|| = \sqrt{{u_x - v_x}^2 + {u_y - v_y}^2 + {u_z - v_z}^2}$$
+$$||w|| = ||u-v|| = \sqrt{{(u_x - v_x)}^2 + {(u_y - v_y)}^2 + {(u_z - v_z)}^2}$$
 
 $$Then, 1)\ ||v||^2 = {v_x}^2 + {v_y}^2 + {v_z}^2$$