2的2次方-2的3次方-......-2的19次方+2的20次方

2^2-2^3-2^4.........-2^19+2^20 =2^2-2^3-2^4.........-2^19+2*2^19=2^2-2^3-2^4.........-2^19+2^19+2^19 =2^2-2^3-2^4.........-2^18+2^19 =2^2-2^3-2^4.........-2^18+2*2^18=2^2-2^3-2^4.........-2^18+2^18+2^18 =2^2-2^3-2^4.........-2^17+2*2^18 .......... =