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方程:x/1*2+x/2*3+x/3*4+```+x/19*20等於19的解是 x[1/1*2+1/2*3+.+1/19*20]=19 x[1-1/2+1/2-1/3+.+1/19-1/20]=19 x[1-1/20]=19 x*19/20=19 x=20
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