(1*2*4+2*4*8+.n*2n*4n)/(1*4*7+2*8*14+...n*4n*7n)
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![(1*2*4+2*4*8+.n*2n*4n)/(1*4*7+2*8*14+...n*4n*7n)](/uploads/image/z/6999252-60-2.jpg?t=%281%2A2%2A4%2B2%2A4%2A8%2B.n%2A2n%2A4n%29%2F%281%2A4%2A7%2B2%2A8%2A14%2B...n%2A4n%2A7n%29)
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(1*2*4+2*4*8+.n*2n*4n)/(1*4*7+2*8*14+...n*4n*7n)
(1*2*4+2*4*8+.n*2n*4n)/(1*4*7+2*8*14+...n*4n*7n)
(1*2*4+2*4*8+.n*2n*4n)/(1*4*7+2*8*14+...n*4n*7n)
(1*2*4+2*4*8+.n*2n*4n)/(1*4*7+2*8*14+...n*4n*7n)
=(2*4)(1^3+2^3+...+n^3)/(4*7)*(1^3+2^3+...+n^3)
=8/28
=2/7