计算:(1-1/2²)(1-1/3²)(1-1/4²)·…·(1-1/99²)(1-1/100²)
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(1-1/2²)(1-1/3²)(1-1/4²)·…·(1-1/99²)(1-1/100²)
=(1+1/2)(1-1/2)(1+1/3)(1-1/3)...(1+1/99)(1-1/99)(1+1/100)(1-1/100) (平方差公式)
=(3/2 ×1/2) ×(4/3×2/3)...(100/99×98/99)×(101/100×99/100)
=(3/2×4/3×...×100/99×101/100)(1/2×2/3×...×98/99×99/100) (前后分开)
=101/2× 1/100
=101/200
=(1+1/2)(1-1/2)(1+1/3)(1-1/3)...(1+1/99)(1-1/99)(1+1/100)(1-1/100) (平方差公式)
=(3/2 ×1/2) ×(4/3×2/3)...(100/99×98/99)×(101/100×99/100)
=(3/2×4/3×...×100/99×101/100)(1/2×2/3×...×98/99×99/100) (前后分开)
=101/2× 1/100
=101/200
追问
什么前后分开?
追答
=(3/2 ×1/2) ×(4/3×2/3)...(100/99×98/99)×(101/100×99/100)每个括号 前面的组合成一组, 后面的组合成一组
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(1-1/2²)(1-1/3²)(1-1/4²)·…·(1-1/99²)(1-1/100²)
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)......(1-1/99)(1+1/99)(1-1/100)(1+1/100)
=1/2* 3/2* 2/3* 4/3* 3/4* 5/4*......*98/99* 100/99* 99/100* 101/100
=1/2* (3/2* 2/3)* ( 4/3* 3/4)* (5/4*......*98/99)* (100/99* 99/100)* 101/100
=1/2* 101/100
=101/200
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=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)......(1-1/99)(1+1/99)(1-1/100)(1+1/100)
=1/2* 3/2* 2/3* 4/3* 3/4* 5/4*......*98/99* 100/99* 99/100* 101/100
=1/2* (3/2* 2/3)* ( 4/3* 3/4)* (5/4*......*98/99)* (100/99* 99/100)* 101/100
=1/2* 101/100
=101/200
【俊狼猎英】团队为您解答
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2012-09-20
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