log10(x) -> Float
x の常用対数を返します。
例 Math.log10(1) # => 0.0 Math.log10(10) # => 1.0 Math.log10(10**100) # => 100.0
Math.log10(1) # => 0.0 Math.log10(10) # => 1.0 Math.log10(10**100) # => 100.0