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