x^2+10x+20
= x^2+10x+ 25 - 5
= （x+5)^2 - 5 = 0
（x+5)^2 = 5
x+5 = ±√5
x= -5 ±√5不好意思，输错了，是x^2+10x+21=0那就更简单了。
x^2+10x+21
= x^2+10x+ 25 - 4
= （x+5)^2 - 4 = 0
（x+5)^2 = 4
x+5 = ±2
x= -5 ±2
x1 = -3, x2 = -7