Lucy and Lily are twins . They are 13 years old . They look the same . But they have different hobbies . Lucy likes collecting stamps . She has many beautiful stamps . They're from different cities and countries . But Lily likes growing flowers . The flowers are all very beautiful . Lucy and Lily both like reading books . Lucy likes reading storybooks . But Lily likes reading science books . On Sunday , they usually ride bikes to the park . They can play with their friends there . Sometimes their parents go there , too .

“$F=0$”是 “圆$x^2+y^2+Dx+Ey+F=0$经过原点”的（   ）

平面上有n条直线，其中没有两条直线互相平行（即每两条直线都相交），也没有三条或三条以上的直线通过同一点．试求：

看拼音写词。