Hello,
3/9 + 5/2 = (3 * 2)/(9 * 2) + (5 * 9)/(9 * 2) = 6/18 + 45/18 = 51/18
6/7 - 2/3 = (6 * 3)/(7 * 3) - (2 * 7)/(7 * 3) = 18/21 - 14/21 = 4/21
7/4 - 9/5 = (7 * 5)/(4 * 5) - (9 * 4)/(4 * 5) = 35/20 - 36/20 = -1/20
3/8 + 1/7 tu mets sous le dénominateur 56
-7/3 + -1/4 tu mets sous le dénominateur 12
2/11 - 3/10 = (2 * 10)/(11 * 10) - (3 * 11)(11 * 10) = 20/110 - 33/110 = -13/110