Решить примеры: 19/42-5/63 (7/10+3//6+2/3) 2/3-(1/20+2/9)-5/18 1/3+1/4+1/5+1/6+1/7

19/42-5/63=(57-10)126=47/126

(7/10+3/5)-(1/6+2/3)=7/15
7/10+3/5=(7+6)/10=13/10
1/6+2/3=(1+4)/6=5/6
13/10-5/6=(39-25)/30=14/30=7/15

2/3-(1/20+2/9)-5/18=7/60
1/20+2/9=(9+40)/180=49/180
2/3-49/180=(120-49)/180=71/180
71/180-5/18=(71-50)/180=21/180=7/60

1/3+1/4+1/5+1/6+1/7=(140+105+84+70+60)/420=459/420=1 39/420