[tex]\LARGE\leadsto\sf JK=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]
[tex]\LARGE\leadsto\sf \sqrt {(1-1)^2+(9-5)^2}[/tex]
[tex]\LARGE\leadsto\sf \sqrt {(0)^2+(4)^2}[/tex]
[tex]\LARGE\leadsto\sf \sqrt {0+16}[/tex]
[tex]\LARGE\leadsto\sf \sqrt {16}[/tex]
[tex]\LARGE\leadsto\sf JK=4units [/tex]
