6×7＝____

3×6＝____

35＋7＝____

7×2＝____

求1+2+2²+2³+…+2²⁰¹⁶的值，可设S=1+2+2²+2³+…+2²⁰¹⁶，于是2S=2+2²+2³+…+2²⁰¹⁷，因此2S﹣S=2²⁰¹⁷﹣1，所以S=2²⁰¹⁷﹣1．我们把这种求和方法叫错位相减法．仿照上述的思路方法，计算出1+5+5²+5³+…+5²⁰¹⁶的值为（   ）

It’s 9:00 in the evening. The family are at home. Jim’s father sits in a chair. He’s watching TV(看电视). His mother stands near the window. She feeds(喂) the birds—Polly. Polly says, “Thanks! Thanks!” Where is Jim? Oh, he’s behind the door. Kate is looking for(找) him. They play games in Kate’s room.

已知两直线l₁：（a+1）x﹣2y+1=0，l₂：x+ay﹣2=0．当a=____时，l₁⊥l₂．