已知点Pn(an,bn)满足an+1=an·bn+1,bn+1=(n∈N*)且点P1的坐标为(1,-1).
Tim:Yes,I have an e-friend in the US.His name is David.He is 15 years old.He is a____student.
某同学常常牙龈出血,甚至皮肤出现淤血,医生建议他多吃蔬菜水果,他可能缺乏( )
下列与激素作用无关的实例是