配方法:
x²-2根号2 x=3
x²-2根号2 x+ (根号2)²=3+(根号2)²
(x-根号2)²=5
x-根号2=±根号5
x=根号2±根号5
所以x1=根号2+根号5,x2=根号2-根号5
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