计算各种底数的对数
log b (x) = ln(x) / ln(b) — 换底公式
log b (x) = ln(x) / ln(b)
log b (xy) = log b (x) + log b (y) — 乘法法则
log b (xy) = log b (x) + log b (y)
log b (x/y) = log b (x) - log b (y) — 除法法则
log b (x/y) = log b (x) - log b (y)
log b (x n ) = n·log b (x) — 幂法则
log b (x n ) = n·log b (x)