设是各项为正数的无穷数列，A_i是边长为a_i,a_i+1的矩形面积（i=1,2,...），则｛A_n}为等比数列的充要条件为（   ）

    This is my family.My mother is a teacher.She is very friendly.My father is a basketball player.He is tall and strong.My uncle is a cook.My aunt is a nurse.I am a student.I have no job.

设向量$\overrightarrow{a}$、$\overrightarrow{b}$，满足|$\overrightarrow{a}$|=|$\overrightarrow{b}$|=1，$\overrightarrow{a}$•$\overrightarrow{b}$=﹣$\frac{1}{2}$，则|$\overrightarrow{a}$+2$\overrightarrow{b}$|=（　　）