a=lg3/lg2
b=lg7/lg3
log42 56
=lg56/lg42
=(lg7+3lg2)/(lg2+lg3+lg7)
=(lg7/lg3+3lg2/lg3)/(lg2/lg3+1+lg7/lg3)
=(b+3/a)/(1/a+1+b)
=(ab+3)/(1+a+ab)
a=lg3/lg2
b=lg7/lg3
log42 56
=lg56/lg42
=(lg7+3lg2)/(lg2+lg3+lg7)
=(lg7/lg3+3lg2/lg3)/(lg2/lg3+1+lg7/lg3)
=(b+3/a)/(1/a+1+b)
=(ab+3)/(1+a+ab)