后面的（lg2）少了个平方
2lg5 + 2/3lg8 +l g5lg20 + lg²2
=2lg5 + 2lg2 + lg5lg20 + lg²2
=2（lg5+lg2) + lg5(lg5+2lg2) + lg²2
=2 + lg²5 + 2lg5lg2 + lg²2
=2+（lg5+lg2)²
=2+1
=3