f（lga）=√10,
a^( lga -1/2) =√10,两边取对数得：
lg(a^( lga -1/2))=lg√10,
( lga -1/2)lga=1/2
( 2lga -1)lga=1
2lg²a- lga-1=0
lga=1或-1/2
a=10或√10/10.