1)1 1/12 - 1/ 15 = 1 + 1·12 12 - 1 /15 = 13 /12 - 1 /15 = 13·5 12·5 -
15·4 = 65 /60 - 4 /60 = 65 - 4 /60 = 61 /60 = 1·60 + 1 60 = 1 1 /60 .
2)P=(1 1/12+1 1/15)•2=1 1 /12 + 1 /15 = 1 + 1 /12 + 1 /15 = 1 + 1 · 5 12 · 5 + 1 · 4 15 · 4 = 1 + 5 60 + 4 60 = 1 + 5 + 4 60 = 1 + 9 60 = 1 + 3 · 3 20 · 3 = 1 + 3 20 = 1 3 20 .
