15.下列运算正确的是( )
| A. | (-x2)4=x8 | B. | a6÷a2=a3 | C. | a2+a3=a5 | D. | (-a)${\;}^{-1}=\frac{1}{a}$ |
13.
如图,已知一次函数y=kx+b,观察图象回答问题:当kx+b>0,x的取值范围是
( )
( )
| A. | x>2.5 | B. | x<2.5 | C. | x>-5 | D. | x<-5 |
9.在Rt△ABC中,∠C=90°,若AB=6,BC=2,则cosB=( )
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| A. | $\frac{1}{3}$ | B. | 3 | C. | $\frac{2\sqrt{2}}{3}$ | D. | $\frac{3\sqrt{2}}{4}$ |