计算（lg2)3次方+（lg5）3次方+3lg2乘以lg5

(lg2)^3+(lg5)^3+3lg2*lg5
=(lg2+lg5)^3+3lg2*lg5-3(lg2)^2lg5-3lg2(lg5)^2
=1+3lg2*lg5(1-lg2-lg5)
=1+3lg2*lg5(1-lg10)
=1那么已知lga+lgb=2lg（a-2b），怎么求log4a除以b的值