题目

已知抛物线的焦点为F，F关于原点的对称点为P，过F作轴的垂线交抛物线于M，N两点，给出下列三个结论：①必为直角三角形；②直线必与抛物线相切；③的面积为．其中正确的结论是___． 答案：【答案】①②③【解析】对于①，验证是否成立即可得到结论是否正确；对于②，求出直线PM的方程后与抛物线方程联立消去得到关于的二次方程，根据判别式的符号进行验证即可得到结论是否正确；对于③，根据三角形的面积公式求出的面积后进行验证即可．对于①：由题意得抛物线的焦点为∴过F作Lucy is a student in Class Two , Grade Seven . She is eleven years old . She had a beautiful toothbrush（牙刷）. But it was broken last Sunday . She was very sad because not only it was beautiful ,but also it was blue --- her favorite color . So her mother went shopping with her to buy a toothbrush on Sunday afternoon . There are many toothbrushes in the shop . They bought a blue one . There is a blue bird in it . And it is made in Guangzhou . It’s ten yuan . It’s expensive . But it is so beautiful . And she likes it very much . Then they went home . Lucy can brush teeth now . How happy she is !根据短文内容从A、B、C、D四个选项中选出正确的答案。1.How old is Lucy ?A . 8 B . 9 C . 10 D . 112.What color was Lucy’s old toothbrush ? A . Red B . Green C . Yellow D . Blue 3.Who went to buy a toothbrush with her on Sunday afternoon ?A . Her mother . B . Her father C . Her friend D . Herself4.Where is Lucy’s toothbrush from ?A . The US B . China C . Japan D . Singapore5.Which sentence has the same meaning as the underline one ?A . I can’t mind it B . I can’t afford it C . It’s good D . It’s not cheap