log9 4=log2 4/log2 9=1/log2 3=a所以log2 3=1/a2^b=7,则b=log2 7log28 42=log2 (2*3*7)/log2（4*7）=[1+log2 3+log2 7]/[2+log2 7]=[1+1/a+b]/[2+b]log6 27=log3 27/log3 6=3/[1+log3 2]=a则log3 2=（3-a）/alog18 ...