If x is close to (but not equal) to a, then x is either
slightly greater or slightly less than a. We can explore these cases
separately. The statement limx→a+f(x)=ℓ
means
"whenever x is slightly greater than a, then f(x) is close to
ℓ'', while limx→a−f(x)=ℓ
means "whenever x is
slightly less than a, f(x) is close to ℓ."