help! about maths51+52+53+.+100=（答案和计算过程）21+22+23+.+50=?

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help! about maths
51+52+53+.+100=（答案和计算过程）
21+22+23+.+50=?

『(1+100)×100÷2』-『(1+50)×50÷2』(答案自己算)
第2道题也是跟第1道一样 ·····

51+52+53+......+100
=(51+100)+(52+99)+...+(75+76)
=151*25
=3775
21+22+23+......+50
=(21+50)+(22+49)+...+(35+36)
=71*15
=1065

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