2.点D、E、F分别在△ABC的三边BC、AB、AC上,且AD、BF、CE相交于一点M,若$\frac{AB}{BE}+\frac{AC}{CF}=5$,则$\frac{AM}{MD}$=( )
| A. | $\frac{7}{2}$ | B. | 3 | C. | $\frac{5}{2}$ | D. | 2 |
20.下列运算中,正确的是( )
| A. | a2•a4=a8 | B. | (a2)3=a6 | C. | a+a3=a4 | D. | (a+b)(a-b)=a2+b2 |
15.若tan40°=a,则tan50°=( )
0 282971 282979 282985 282989 282995 282997 283001 283007 283009 283015 283021 283025 283027 283031 283037 283039 283045 283049 283051 283055 283057 283061 283063 283065 283066 283067 283069 283070 283071 283073 283075 283079 283081 283085 283087 283091 283097 283099 283105 283109 283111 283115 283121 283127 283129 283135 283139 283141 283147 283151 283157 283165 366461
| A. | $\frac{1}{a}$ | B. | -a | C. | a | D. | 2a |