—The photo album looks old, and some pages are lost. Can you imagine how long the man        it?
—I’m not sure. Probably for many years.

—How long         you        this bicycle?
—For two years.

Sanya is a beautiful city. I          there twice.

I began watching here an hour ago, but now nothing         yet.

Our foreign teacher Mr Green       us English since three years ago.

—Would you like to watch the three-D film Titanic with me?
—Certainly. I don’t mind seeing it again although I         it twice.

—You haven't been to the West Lake, have you?
—         .But I will go there with my parents this summer vacation.

—Hello,may I speak to Mr. Smith?
—Sorry,he isn't in. He           the library.

如图,n+1个边长为2的等边三角形有一条边在同一直线上,设△B₂D₁C₁的面积为S₁,△B₃D₂C₂的面积为S₂,…,△B_n+1D_nC_n的面积为S_n,通过计算S₁,S₂,…,的值,则S_n的表达式为(    )(用含n的式子表示).

如图,梯形ABCD的两底BC=2AD,O为其内部一点,使得△AOD的面积与△BOE的面积之和是4,E是OC的中点.则梯形ABCD的面积是(    )