《2025年时习之暑假衔接七年级数学人教版》

解方程组:
(1)$\left\{\begin{array}{l} 2x+y= 2,\\ 8x+3y= 9;\end{array} \right. $
(2)$\left\{\begin{array}{l} 2x-5y= -21,\\ 4x+3y= 23;\end{array} \right. $
(3)$\left\{\begin{array}{l} 3x+y= 22,\\ 4(x+y)-5(x-y)= 2;\end{array} \right. $
(4)$\left\{\begin{array}{l} \frac {x-2}{3}-\frac {y+1}{2}= 2,\\ \frac {2x+1}{4}+\frac {y-6}{3}= 3;\end{array} \right. $
(5)$\left\{\begin{array}{l} x+y+z= 26,\\ x-y= 1,\\ 2x+z-y= 18.\end{array} \right. $

答案：
(1)$\left\{\begin{array}{l} x=\frac {3}{2},\\ y=-1.\end{array}\right. $
(2)$\left\{\begin{array}{l} x=2,\\ y=5.\end{array}\right. $
(3)$\left\{\begin{array}{l} x=7,\\ y=1.\end{array}\right. $
(4)$\left\{\begin{array}{l} x=9.5,\\ y=0.\end{array}\right. $
(5)$\left\{\begin{array}{l} x=10,\\ y=9,\\ z=7.\end{array}\right. $

1.按如图所示的程序计算,若开始输入的n值为$\sqrt {2}$,则最后输出的结果是( )


A.2
B.$5-\sqrt {2}$
C.$6+\sqrt {2}$
D.$1+\sqrt {2}$

答案： C

2.已知$9.96^{2}\approx 99.2,9.97^{2}\approx 99.4,9.98^{2}\approx 99.6$,则$\sqrt {996000}$的个位数字约等于( )

A.0
B.4
C.6
D.8

答案： D

3.已知$\sqrt [3]{68.8}\approx 4.098$,且$\sqrt [3]{-x}\approx 40.98$,则$x= $____.

答案： -68800

4.计算下列各式的值:
$\sqrt {9^{2}+19}= $____;
$\sqrt {99^{2}+199}= $____;
$\sqrt {999^{2}+1999}= $____;
$\sqrt {9999^{2}+19999}= $____.
观察所得结果,总结存在的规律,应用得到的规律可得$\sqrt {\underbrace {99... 9^{2}}_{2025个9}+\underbrace {199... 9}_{2025个9}}= $____.

答案： 10 100 1000 10000 $10^{2025}$