[tex]\text{Rownanie prostej AB}\\\left \{ {{8=-4a+b /*2} \atop {3=8a+b}} \right. \\+\left \{ {{16=-8a+2b} \atop {3=8a+b}} \right. \\16+3=2b+b\\19=3b /:3\\b=\frac{19}3\\\\3=8a+\frac{19}3\\\frac93-\frac{19}3=8a\\-\frac{10}3=8a /*\frac18\\-\frac{10}3*\frac18=a\\a=-\frac53*\frac14\\a=-\frac5{12}\\\\y=-\frac5{12}x+\frac{19}3 /*12\\12y=-5x+76\\\underline{5x+12y=76}[/tex]
[tex]\text{Rownanie prostej BC}\\\left \{ {{-3=8a+b /*(-1)} \atop {6=5a+b}} \right. \\+\left \{ {{3=-8a-b} \atop {6=5a+b}} \right. \\3+6=-8a+5a\\9=-3a /:(-3)\\a=-3\\\\6=5*(-3)+b\\6=-15+b /+15\\21=b\\\\y=-3x+21\\\underline{3x+y=21}[/tex]
[tex]\text{Rownanie prostej CD}\\+\left \{ {{6=5a+b} \atop {0=-5a+b}} \right. \\6+0=b+b\\6=2b /:2\\3=b\\\\0=-5a+3\\5a=3 /:5\\a=\frac35\\\\y=\frac35x+3 /*5\\5y=3x+15\\\underline{-3x+5y=15}\\\underline{3x-5y=-15}[/tex]
[tex]\text{Rownanie prostej AD}\\\left \{ {{8=-4a+b} \atop {0=-5a+b /*(-1)}} \right. \\+\left \{ {{8=-4a+b} \atop {0=5a-b}} \right. \\8+0=-4a+5a\\8=a\\\\0=-5*8+b\\0=-40+b\\b=40\\\\y=8x+40\\\underline{-8x+y=40}\\\underline{8x-y=-40}[/tex]