(20-x)(30-x)=551
x² - 50x + 600 -551 = 0
x² - 50x + 49 = 0
(x-1)(x-49) = 0
x = 1 或 x = 49
x[(7-2x)/2]=2
7x - 2x² = 4
2x² -7x + 4 = 0
x = (7 -√17)/4 或 x = (7 +√17)/4