20.下列等式从左到右的变形,属于因式分解的是( )
| A. | a(x-y)=ax-ay | B. | x2+2x+1=x(x+2)+1 | C. | x3-x=x(x+1)(x-1) | D. | (x+1)(x+3)=x2+4x+3 |
18.已知方程组$\left\{\begin{array}{l}{x+2y=k}\\{2x+y=2}\end{array}\right.$的解满足x+y=2,则k的算术平方根为( )
| A. | 4 | B. | -2 | C. | -4 | D. | 2 |
13.在人体血液中,红细胞的直径约为7.7×10-4cm,7.7×10-4用小数表示为( )
0 285032 285040 285046 285050 285056 285058 285062 285068 285070 285076 285082 285086 285088 285092 285098 285100 285106 285110 285112 285116 285118 285122 285124 285126 285127 285128 285130 285131 285132 285134 285136 285140 285142 285146 285148 285152 285158 285160 285166 285170 285172 285176 285182 285188 285190 285196 285200 285202 285208 285212 285218 285226 366461
| A. | 0.000077 | B. | 0.00077 | C. | -0.00077 | D. | 0.0077 |