bai toan nay tuoc dang dat an phu
goi t la x^2 -5x( hoac ban co cthe dat la x^2 -5x + 1 deu dc nha)
=) t(t+1) -12=0
(=)t^2+t-12=0
(=)(roi dug may tinh bam tinh nhu may bai tach bt)
........
(t+4)(t-3)=0
t+4=0 hay t-3=0
t=-4 hay t=3
thay t=x^2-5x ta dc
x^2-5x=-4 hay x^2=3
giai bt........