Problem $4(20$ points$)$

Solve the following mathematical models graphically.

a$)$

MaxZ$=x_1+3x_2$

$x_1+2x_{2}2$

$-x_1+2x_{2}4$

$x_1,x_{2}0$

b$)$

MaxZ$=x_1+x_2$

$-2x_1+3x_{2}6$

$x_{2}1$

$x_1,x_{2}0$