(lg2)^2+lg5*lg20-1
=(lg2)^2+lg5*lg(2*10)-1
=(lg2)^2+lg5*(lg2+lg10)-1
=(lg2)^2+lg5*lg2+lg5*lg10-1
=lg2*(lg2+lg5)+lg5*1-1
=lg2*1+lg5-1
=(lg2+lg5)-1
=1-1
=0
(lg2)^2+lg5*lg20-1
=(lg2)^2+lg5*lg(2*10)-1
=(lg2)^2+lg5*(lg2+lg10)-1
=(lg2)^2+lg5*lg2+lg5*lg10-1
=lg2*(lg2+lg5)+lg5*1-1
=lg2*1+lg5-1
=(lg2+lg5)-1
=1-1
=0