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1X2+2X3X4+3X4X5X6+......+10X11X12X13......X20的...

通项为:n(n+1)=n^2+n 因∑n^2=n(n+1)(2n+1)/6 ∑n=n(n+1)/2 所以:1x2+2x3+3x4+……+19x20 =19*(19+1)*(2*19+1)/6+19*(19+1)/2 =19*20*39/6+19*20/2 =19*10*13+19*10 =190*14 =2660

2.65252828598*10^32

int i=1,s=0;do{s=s+i*(i+1)*(i+2);while(i

【解答】原式×3可以得到如下变形 1×2×3+2×3×3+3×4×3+……+19×20×3 =1×2×3+2×3×(4-1)+3×4×(5-2)+……+19×20×(21-18) =1×2×3-1×2×3+2×3×4-2×3×4+3×4×5+……-18×19×20+19×20×21 =19×20×21 由此可以知道原式=19×20×21÷3=2660

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