ax^2+bx+c=0ax^2+bx=-cx^2+bx/a=-c/ax^2+bx/a+b^2/4a^2=b^2/4a^2-c/a（x+b/2a）^2=b^2/4a^2-4ac/4a^2（x+b/2a）^2=（b^2-4ac）/4a^2两边开根号x+b/2a=土根号（b^2-4ac）/2ax=土根号（b^2-4ac）/2a-b/2ax=（-b土根号...