加減看一下

aX^2+bX+c
=a{X^2+[b/a]X}+c
=a{X^2+2[b/(2a)]X}+c
=a{X^2+2[b/(2a)]X+[b/(2a)]^2}+c-a[b/(2a)]^2
=a{X+[b/(2a)]}^2+c-a[b/(2a)]^2
=0
=>a{X+[b/(2a)]}^2=a[b/(2a)]^2-c
=>{X+[b/(2a)]}^2=[b/(2a)]^2-c/a
________________=b^2/[4a^2]-c/a
________________=b^2/[4a^2]-[c*4a]/[a*4a]
________________=b^2/[4a^2]-[4ac]/[4a^2]
________________=(b^2-4ac)/[4a^2]
=>{X+[b/(2a)]}=±√{(b^2-4ac)/[4a^2]}
______________=±√(b^2-4ac)/[2a]
=>X=-[b/(2a)]±√(b^2-4ac)/[2a]
____={-b±√(b^2-4ac)}/[2a]
判別式就是根號裡面的東西：b^2-4ac