含有30°角的直角三角形三边长的比值是______；含有45°角的直角三角形三边长的比值是______．
在30°角的直角三角形中，设30°角对的直角边为a，则斜边为2a，运用勾股定理另一条直角边为
=(2a)2-a2
a．3
∴三边长的比值为：a：
a：2a=1：3
：2．3
在45°角的直角三角形中，设直角边为b，则斜边为
=b2+b2
b，2
∴三边的比值为：b：b：
b=1：1：2
．2
故答案是：1：
：2． 1：1：3
．2
Directions: There are 10 blanks in the following passage. For each numbered blank, there are 4 choices marked A, B, C and D.
| Historians tend to tell the same joke when they are describing history education in America. It’s the one (51) the teacher standing in the schoolroom door (52) goodbye to students for the summer and calling after them. "By the way, we won World War II." The problem with the joke, of course, is that it’s not funny. The recent surveys on (53) illiteracy are beginning to numb. nearly one third of American 17-year-olds cannot even (54) which countries the United States fought against in that war. One third have no idea when the Declaration of Independence was (55) . One third thought Columbus reached the New World after 1750. Two thirds cannot correctly (56) the Civil War between 1850 and 1900. Even when they get the answers right, some are just guessing. Unlike math or science, ignorance of history cannot be (57) connected to toss of international competitiveness. But it does affect our future (58) a democratic nation and as individuals. The good news is that there is growing agreement on what is wrong with the (59) of history and what needs to be done to fix it. The steps are tentative (尝试性的) (60) yet to be felt in most classrooms. |
A．shaking
B．waving
C．nodding
D．speaking