在数学学习过程中 ,由于我们对有关知识理解上的偏差 ,或思维的不严谨 ,或知识应用的疏漏 ,常常使我们走进这样或那样的误区 ,为防止此类错误的出现 ,现举几例帮助同学们走出解此类题的误区 .例 1　已知f(x) =ax2 -c ,且 - 4≤f( 1)≤ - 1,- 1≤f( 2 )≤ 5,求 f( 3)的取值范围 In the process of mathematics learning, due to deviations in our understanding of relevant knowledge, or the rigorous thinking, or omissions in the application of knowledge, we often get into this or that misunderstanding. To prevent the emergence of such mistakes, Examples help students get out of the solution to this type of problem. Example 1 Given that f(x) = ax2 -c, and -4≤f(1)≤ - 1, - 1≤f(2)≤5, find f( 3) Value range