y=ax^2,y'=2ax
y=lnx,y'=1/x
2ax=1/x
x=√(1/2a)分别代入y=ax^2与y=lnx,则
ln√(1/2a)=1/2
a=1/2e
切点为(√e,1/2)
y-1/2=(x-√e)/√e