√4＜√7＜√9
2＜√7＜3
√7的小数部分是：√7-2
2+√7的小数部分也是：√7-2
a=√7-2
a(a+4)
=(√7-2)(√7-2+4)
=(√7-2)(√7+2)
=7-4
=3