换底公式 我喜欢ln 所以 log2(9) =ln9/ln2 =ln(3^2)/ln2 =2*ln3/ln2 log3(4) =ln4/ln3 =ln(2^2)/ln3 =2*ln2/ln3 log2(9)*log3(4) =2*ln3/ln2*2*ln2/ln3 =2*2*(ln3/ln2*ln2/ln3) =4*1 =4
