已知（ax+1）ⁿ=a₀+a₁x+a₂x²+…+a_nxⁿ，若a₁=4，a₂=7，则a值为______．

基本语法

1. I was ill ,so I        __(不得不)stay at home today.

2. You____   (不必)return that book to me at once.

3. We        _ (可能去)to visit my father's farm during the summer vacation.

4. You    ___  (可以)call him. He   __   (不可能)do anything else this afternoon.

5. ______(或许)she will invite you to her party ,too.