[(lg5)^2--(lg2)^2]/(lg25--lg4)
=[(lg5+lg2)(lg5-lg2)]/(lg5^2-lg2^2)
=[(lg5+lg2)(lg5-lg2)]/(2lg5-2lg2)
=[(lg5+lg2)(lg5-lg2)]/2(lg5-lg2)
=(lg5+lg2)/2