设甲单独做要x天,乙单独做要y天,丙单独做要z天
a(1/x)=1/y+1/z=>(a+1)(1/x)=1/x+1/y+1/z=>1/(a+1)=(1/x)/(1/x+1/y+1/z)
b(1/y)=1/x+1/z=>(b+1)(1/y)=1/x+1/y+1/z=>1/(b+1)=(1/y)/(1/x+1/y+1/z)
c(1/z)=1/x+1/y=>(c+1)(1/z)=1/x+1/y+1/z=>1/(c+1)=(1/z)/(1/x+1/y+1/z)
三式两边分别叠加,得
1/(a+1)+1/(b+1)+1/(c+1)=1