Mr. Jackson was tired of living in his old house in the countryside and wanted to sell it and buy a better one. He tried to sell it for a long time, but was not successful, so at last he decided to go to an estate agent（房产经纪人）.

    The agent advertised the house straight away, and a few days later, the owner saw a very beautiful photograph of his house, with a wonderful description of his garden in an expensive magazine.

    Having read the advertisement through, the house owner hurried to phone the estate agent, saying, “I'm sorry, Mr. Jones, but I've decided not to sell my house after all.” “Why?” the agent asked in a surprised voice. “Because from the advertisement in that magazine I can see it's just the kind of house I've wanted to live in all my life.”

已知f（x）是定义在R上且以4为周期的奇函数，当x∈（0，2）时，f（x）=ln（x²﹣x+b），若函数f（x）在区间﹣2，2上的零点个数为5，则实数b的取值范围是____ 

$\frac{4}{25}$的算术平方根是（）

如图示，在匀强磁场中有一U形导线框abcd，线框处于水平面内，磁场与线框平面垂直，R为一电阻. ef为垂直于ab、cd的一根导体杆，可以在ab、cd上无摩擦地滑动. 杆ef及线框的电阻均不计. 开始时，给ef一个向右的初速度v₀，则 (    )