（1）x=3；y=a²=4;
a=±2;
∵a＞0;
∴a=2;
(2)lg1/100=-2;
f(-2)=a^(-3)=1/a^3;
f(-1/2)=a^(-3/2)=1/a^(3/2)=1/√a³;
∴0＜a＜1时；f(lg1/100)＜f(-1/2);
a＞1时；f(lg1/100)＞f(-1/2);
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