1-1/2+1/3-1/4........-1/40=N/M
=1+1/2+1/3+1/4......+1/40-2*(1/2+1/4+1/6.....+1/40)
=1+1/2+1/3+.....+1/40 - (1+1/2+1/3+1/6...+1/20)
=1/21+1/22+1/23.......+1/40
=(1/21+1/40)+(1/22+1/39)+.......(1/30+1/31)
=61/(21*40) + 61/(22*39) + ..... +61/(30*31)
=61 * ( 1/(21*40)+1/(22*39)+...+1/(30*31) )
=61 * P/M = N/M
所以N為61的倍數
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