主要利用0.25X4=1原式=0.25^4 X9=0.25^4 X4^4 X9/(4^4)=9/4^4=9/256
0.25×0.25×0.75×0.75=1/4x1/4x3/4x3/4=9/(16x16)=9/256=256分之9