18.(1)f(x)=-x^2-2x,x>=0;
f(x)æ¯å¥å½æ°ï¼
â´x<0æ¶f(x)=-f(-x)=-[-(-x)^2-2(-x)]=-(-x^2+2x)=x^2-2x.
(2)f(x)=-x^2+ax=-(x-a/2)^2+a^2/4,x>=0为åå½æ°ï¼
â´a/2<=0,a<=0,
f(x)æ¯å¥å½æ°ï¼
â´a<=0æ¶f(x)å¨Rä¸æ¯åå½æ°ï¼
â´açåå¼èå´æ¯(-âï¼0].
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