立体几何是概念精练明晰,推理逻辑严密, 空间想象丰富的学科．如果在立体几何的学习 中,概念不清,逻辑不严,想象不对,思考不周,很 容易出现解题失误,下面将举例说明之． 例1 在空间四边形ABCD中,AB、CD 所成角为60°,且AB=8,CD=6,E、F分别是 BD、AC的中点,求线段EF的长． 3. The three-dimensional geometry is a discipline with concise conception, strict reasoning logic, and rich spatial imagination. If you are learning in solid geometry, the concept is unclear, the logic is lax, the imagination is not correct, you have poor thinking, and it is easy to make mistakes. Here is an example to illustrate. Example 1 In the space quadrilateral ABCD, the angle formed by AB and CD is 60°, and AB=8, CD=6. E and F are the midpoints of BD and AC, respectively, and the length of the line EF is required.