a = ㏒0.7（0.8）= ln0.8/ln0.7 = 0.62562...
b = ㏒1.1（0.9）= ln0.9/ln1.1 = -1.1054...
lnc = 0.9ln1.1 = 0.085779182...
∴ c = 1.1^0.9 = e^0.085779182... = 1.0895684.
∴ c > a > b