1、2x+y=54 y=54-2x
xy=72
x(54-2x)=72
54x-2x²=72
2x²-54x=-72
x²-27x=-36
x²-27x+(27/2)²=729/4-144/4
(x-27/2)²=585/4
x-27/2=±3√65/2
x=27/2±3√65/2
2、5x+3y=54 3y=54-5x y=(54-5x)/3
xy=72
x(54-5x)/3=72
54x-5x²=216
5x²-54x=-216
x²-54x/5=-216/5
x²-54x/5+(27/5)²=729/25-1080/25
(x-27/5)²=-351/25
实数范围内无解
x-27/5=±3√39i/5
x=27/5±3√39i/5
3、5x+2y=54 2y=54-5x y=(54-5x)/2
xy=72
x(54-5x)/2=72
54x-5x²=144
5x²-54x=-144
x²-54x/5=-144/5
x²-54x/5+(27/5)²=729/25-720/25
(x-27/5)²=9/25
x-27/5=±3/5
x=27/5±3/5
x1=27/5+3/5=30/5=6
x2=27/5-3/5=24/5