1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90 calcul d'abords 1/2-1/3 puis 1/3-1/4 et 1/4-1/5
1/6+1/12=2/12+1/12 = 3/12= 6/24
1/20+1/30=30/600+20/600 = 50/600= 5/60 = 1/12=2/24
1/42+1/56=56/2352+42/2352=98/2352=49/1176 = 7/168=1/24
1/72+1/90=90/6480+72/6480=162/6480= 1/40
6/24+2/24+1/24+1/40=9/24+1/40
1/40+9/24= 24/960+360/960
=384/960 (divier par 4/4)
=96/240 (diviser par 3)
=32/80 (diviser par 4)
=8/20 (diviser par 4)
=2/5
