-5x-2x平方-1=0
2x²+5x+1=0
x²+5x/2+1/2=0
x²+5x/2+1/2
=(x²+5x/2+25/16)-25/16+1/2
=(x+5/4)²-17/16
=(x+5/4+√17/4)(x+5/4-√17/4)
=[x+(5+√17)/4][x+(5-√17)/4]=0
x1=-(5+√17)/4
x2=(√17-5)/4]