log2f(a)=2
f(a)=a²-a+k=4
k=-a²+a+4
f(log2(a))=[log2(a)]²-log2(a)+k=k
log2(a)[log2(a)-1]=0
log2(a)=0,log2(a)=1
a≠1,所以log2(a)=1
a=2
k=-a²+a+4=2
log2f(a)=2
f(a)=a²-a+k=4
k=-a²+a+4
f(log2(a))=[log2(a)]²-log2(a)+k=k
log2(a)[log2(a)-1]=0
log2(a)=0,log2(a)=1
a≠1,所以log2(a)=1
a=2
k=-a²+a+4=2