log10(∞) = ?
Since infinity is not a number, we should use limits:
The limit of the logarithm of x when x approaches infinity is infinity:
lim log10(x) = ∞
The opposite case, the logarithm of minus infinity (-∞) is undefined for real numbers, since the logarithmic function is undefined for negative numbers:
lim log10(x) is undefined
x → -∞