2+4+6+...+2n-2+2n
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![2+4+6+...+2n-2+2n](/uploads/image/z/2689271-71-1.jpg?t=2%2B4%2B6%2B...%2B2n-2%2B2n)
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2+4+6+...+2n-2+2n
2+4+6+...+2n-2+2n
2+4+6+...+2n-2+2n
原式=(2+2n)n/2=n(n+1)=n²+n
2+4+6+8+。。。。。。+(2n-2)+2n
=2×[1+2+3+4+......+(n-1)+n]
=2×(1+n)×n÷2
=n(n+1)