19.直角三角形的两直角边为a,b,斜边上的高为h,则下列各式中总是成立的是( )
| A. | $\frac{1}{{a}^{2}}+\frac{1}{{b}^{2}}=\frac{1}{{h}^{2}}$ | B. | $\frac{1}{a}+\frac{1}{b}=\frac{1}{h}$ | C. | a2+b2=2ah | D. | $\frac{1}{a}+\frac{1}{b}=\frac{2}{h}$ |
18.已知$\left\{\begin{array}{l}{x+2y=4k}\\{2x+y=2k+1}\end{array}\right.$且0<y-x<1,则k的取值范围是( )
| A. | -1$<k<-\frac{1}{2}$ | B. | 0$<k<\frac{1}{2}$ | C. | 0<k<1 | D. | $\frac{1}{2}$<k<1 |
15.
如图,已知∠1=∠B,∠2=∠C,则下列结论不成立的是( )
0 284867 284875 284881 284885 284891 284893 284897 284903 284905 284911 284917 284921 284923 284927 284933 284935 284941 284945 284947 284951 284953 284957 284959 284961 284962 284963 284965 284966 284967 284969 284971 284975 284977 284981 284983 284987 284993 284995 285001 285005 285007 285011 285017 285023 285025 285031 285035 285037 285043 285047 285053 285061 366461
| A. | AD∥BC | B. | ∠B=∠C | C. | ∠2+∠B=180° | D. | AB∥CD |