题目

A video of a one—arm Chinese boy playing basketball recently went viral (走红) on the Internet in China. In the video, the young boy was playing with his great basketball skills, which even drew the attention of NBA super star Steven Curry.I didn't even realize he had only one arm when I first watched the video, said one person after watching the video.The one—arm 13—year—old is a country boy from Yunfu City, south China's Guangdong Province. His name is Zhang Jiacheng. At the age of five,he lost his right arm after an accident at an oil factory. Zhang had to get used to eating and g若函数f（x）=-x2+2ax+1+a在区间[0，2]上最大值为5，求实数a的值．