题目

阅读理解My mother and her best friend Martha were in their mid-80s whenthey saw each other for the last time.They had been friends since they were 18, both of them students atWesleyan College in Macon, Georgia. My mother was the dreamy one, who loved toread and dream herself as an actress. Martha, however, was more practical. I'mnot sure what they had in common except that they trusted each other, helpedeach other, and stayed in touch even when life separated them.Martha spent most of her adulthood in Atlanta, where she raisedthree kids. My mother, meanwhile, raised eight children, and mov如图，AB为⊙O的切线，B为切点．若∠A=30°，AO=6，则OB=________．