已知为等差数列的前项和，，则为（ ）
Nice to meet you. A.Nice to meet you, too. B.Fine, thanks. C.How are you?
How old are you? A. I’m ten. B. Fine, thank you. C. How do you do?
This is an apple. It’s ______red. A. a B. an C. /
Is that ____eraser? A. your B. you C. yours
This is _____orange. A. an B. a C. this
为等差数列
的前
项和，
，则
为（ ）
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