comment fait on pour additionner 3 fraction 1/12 plus 1/20 plus 1/35

1/12 + 1/20 + 1/35
= 35/420 + 21/420 + 12/420
= 68/420 = 17/105

1/12 + 1/20 + 1/35
= 1 x 35 / 12 x 35 + 1 x 21 / 20 x 21 + 1 x 12 / 35 x 12
= 35/420 + 21/420 + 12/420
= 68/420
= 4 x 17 / 4 x 105
= 17/105