4.下列各式中,属于二元一次方程的是( )
| A. | x2+y=0 | B. | $\frac{x+y}{3}$-2y=1 | C. | x=$\frac{2}{y}$+1 | D. | y+$\frac{1}{2}$x |
1.立方根等于它本身的有( )
| A. | -1,0,1 | B. | 0,1 | C. | 0,-1 | D. | 1 |
20.若不等式ax+x>1+a的解集是x<1,则a必须满足的条件是( )
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| A. | a<-1 | B. | a<1 | C. | a>-1 | D. | a>1 |