20²-19²+18²-17²+.+2²-1² (过程)
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![20²-19²+18²-17²+.+2²-1² (过程)](/uploads/image/z/201403-19-3.jpg?t=20%26%23178%3B-19%26%23178%3B%2B18%26%23178%3B-17%26%23178%3B%2B.%2B2%26%23178%3B-1%26%23178%3B+%EF%BC%88%E8%BF%87%E7%A8%8B%EF%BC%89)
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20²-19²+18²-17²+.+2²-1² (过程)
20²-19²+18²-17²+.+2²-1² (过程)
20²-19²+18²-17²+.+2²-1² (过程)
根据a^2-b^2=(a+b)(a-b)
上式=(20+19)(20-19)+(18+17)(18-17)······+(2+1)(2-1)
=(20+19)+(18+17)······+(2+1)
=20*(20+1)/2
=210