a)
3/70 + 2/7 - 12/15 =
= 3/70 + 2/7 - 4/5 =
= 3/70 + 2×10/70 - 4×14/70 =
= 3/70 + 20/70 - 56/70 =
= - 33/70
b)
1/2 - 1/3 +1/4 - 1/5 =
= 30/60 - 20/60 + 15/60 - 12/60 =
= (30 - 20 + 15 - 12)/60 =
= 13/60
c)
4/3 -(-3/7)+11/21 -1 =
= 4/3 + 3/7 + 11/21 - 1 =
= 4×7/21 + 3×3/21 + 11/21 - 21/21 =
= 28/21 + 9/21 + 11/21 - 21 /21 =
= (28 + 9 + 11 - 21)/21 =
= 27/21 =
= 9/7
