Solve in excel please $12\mathrm{.}16$ Solve each of the following systems of simultaneous equations. In each case,

verify that the solution is correct using the method described in Prob. $12\mathrm{.}14.$

$($a$)$

$15x_1+20x_2=25$

$5x_1+10x_2=12$

$($b$)$

$4x_1-2x_2+x_3=6$

$x_1+3x_2-7x_3=34$

$-2x_1-5x_3=14$

$x_1+x_2+5x_3-12x_4=29$

$x_2+16x_4=21$

$($c$)$

$x_2,+16x_4=21$

$7x_1+2x_2,-12x_4=5$

$,x_3-,x_4=6$