2-2²-2³-……2^18-2^19+2^20等于多少

原式=2-2^2-2^3-……-2^19(1-2)
=2-2^2-2^3-……-2^18+2^19
=2-2^2-2^3-……-2^17-2^18(1-2)
=2-2^2-2^3-……-2^17+2^18
=2-2^2+2^3
=2-4+8
=6