阅读理解
December 21st is Becky's fourteenth birthday. When she gets home after school, she sees her parents in the living room. They both say to her, “Happy birthday, dear! We have a present for you. Go and look for it in your bedroom. ”
Becky runs to her bedroom. There is a red box on her desk. There is a card in it. The girl takes it out and reads，“Dear Becky, I'm your present. My first three letters are in the word（单词）'come'. My last two letters are in the word 'water'. My middle（中间的） three letters are in the word 'put'. What am I?”
Becky thinks hard and then says,“Thank Dad and Mom. I know what it is, but where is it?”
Her father and mother tell her to go to her study. There, on the desk she finds her new present.
所在平面内一点，D为BC边中点，且
，那么
