题目

When other nine-year-old kids were playing games, she was working at a petrol station. When other teens were studying or going out, she struggled to find a place to sleep on the street. But she overcame these terrible setbacks to win a highly competitive scholarship and gain entry to Harvard University. And her amazing story has inspired a movie, “Homeless to Harvard: The Liz Murray Story”, shown in late April. Liz Murray, a 22-year-old American girl, has been writing a real-life story of willpower and determination. Liz grew up in the shadow of two drug-addicted parents. There was 已知a，b都是正实数，且满足log4（2a+b）=log2ab，则2a+b的最小值为 ．