《2025年暑假Happy假日七年级理科五四制》

1. 下列是三元一次方程组的是 (D)
A. $\left\{\begin{array}{l} 2x=5,\\ x^{2}+y=7,\\ x+y+z=6\end{array}\right. $
B. $\left\{\begin{array}{l} \frac {3}{x}-y+z=-2,\\ x-2y+z=9,\\ y=-3\end{array}\right. $
C. $\left\{\begin{array}{l} x+y-z=7,\\ xyz=1,\\ x-3y=4\end{array}\right. $
D. $\left\{\begin{array}{l} x+y=2,\\ y+z=1,\\ x+z=9\end{array}\right. $

答案： D

2. 解方程组$\left\{\begin{array}{l} 2x-y+3z=3,\\ -4x+y+2z=11,\\ 5x+y+7z=1,\end{array}\right. $若要使运算简便,消元应选 (B)
A. 先消x
B. 先消y
C. 先消z
D. 先消常数项

答案： B

3. 三元一次方程组$\left\{\begin{array}{l} x-y=6,\\ x+z=4,\\ 2x-y+z=10\end{array}\right. $的解的个数为 (A)
A. 无数多个
B. 1
C. 2
D. 0

答案： A

4. 解方程组$\left\{\begin{array}{l} 4x+3z=9,\\ 2x-6y+4z=5,\\ 3x+2y+z=8\end{array}\right. $较简便的方法是 (B)
A. 先消z,再解$\left\{\begin{array}{l} 2x-6y=-15,\\ 19x+9y=8\end{array}\right. $
B. 先消z,再解$\left\{\begin{array}{l} 11x+3y=9,\\ 10x+14y=27\end{array}\right. $
C. 先消y,再解$\left\{\begin{array}{l} 4x+3z=9,\\ 11x+7z=29\end{array}\right. $
D. 先消x,再解$\left\{\begin{array}{l} 22y+2z=61,\\ 66y-38z=-33\end{array}\right. $

答案： B

5. 已知x,y同时满足下列三个等式:$4x+y=a+1,5x+2y=a,3x-2y=7a$,则a的值为 (C)
A. -2
B. -1
C. 1
D. 2

答案： C