三角形一边长为 ，另两边长是方程 的两实根，则这是一个（   ）.

    One day in the 1 class everyone is busy drawing pictures, but Xiao Ming is doing 2. When the bell rings for break（课间休息）, Xiao Ming gives his 3 to the teacher.

    The teacher looks at it and gets 4. "Why do you give me a piece of paper with nothing on it, Xiao Ming?" 5 the teacher.

    "I'm sorry, sir," says Xiao Ming. "I 6 a dog and some 7 on the paper."

    "Where's the dog and where's the bread?" asks the teacher.

    "I draw the bread 8. When I finish drawing the dog, he9 the bread. When I give it to you, he 10 away," Xiao Ming answers.

①a₁=1；②当n≥2时，|a_i﹣a_i+1|≤2（i=1，2，…，n﹣1）．

记这样的数列个数为f（n）．

（ 1）写出f（2），f（3），f（4）的值；

（ 2）证明f（2018）不能被4整除．