18.下列运算正确的是( )
| A. | x5•x=x5 | B. | x5-x2=x3 | C. | (-y)2 (-y)7=y9 | D. | -y3•(-y)7=y10 |
16.下列运算正确的是( )
| A. | a3+a3=26a | B. | 3a-2a=a | C. | 3a2b-4b2a=-a2b | D. | (-a)2=-a2 |
10.
如图,在?ABCD中,点E在DC上,BE与AC相交于点F,若$\frac{DE}{EC}$=$\frac{1}{2}$,则$\frac{BF}{BE}$的值为( )
0 282982 282990 282996 283000 283006 283008 283012 283018 283020 283026 283032 283036 283038 283042 283048 283050 283056 283060 283062 283066 283068 283072 283074 283076 283077 283078 283080 283081 283082 283084 283086 283090 283092 283096 283098 283102 283108 283110 283116 283120 283122 283126 283132 283138 283140 283146 283150 283152 283158 283162 283168 283176 366461
| A. | $\frac{3}{5}$ | B. | $\frac{3}{4}$ | C. | $\frac{4}{5}$ | D. | $\frac{2}{3}$ |