Вычислите: а)1/4+4/7+9/28 б)1/5+3/10+7/20 в)3/20+7/30+2/40 г)17/20+23/30+11/60 д) 3/20+7/30+9/59 е) 7/40+11/70+13/30

Приведем дроби к общему знаменателю и вычислим значения выражений:

1) 1/4 + 4/7 + 9/28 = (1 * 7)/(4 * 7) + (4 * 4)/(7 * 4) + 9/28 = 7/28 + 16/28 + 9/28 = (7 + 16 + 9)/28 = (7 + 25)/28 = 32/28 = 1 4/28 = 1 1/7;

2) 1/5 + 3/10 + 7/20 = (1 * 4)/20 + (3 * 2)/20 + 7/20 = 4/20 + 6/20 + 7/20 = (4 + 6 + 7)/20 = 17/20;

3) 3/20 + 7/30 + 2/40 = 3/20 + 7/30 + 1/20 = 4/20 + 7/30 = 1/5 + 7/30 = 6/30 + 7/30 = 13/30;

4) 17/20 + 23/30 + 11/60 = 51/60 + 46/60 + 11/60 = 51/60 + 57/60 = 108/60 = 1 48/60 = 1 4/5;

5) 3/20 + 7/30 + 9/60 = 9/60 + 14/60 + 9/60 = 23/60 + 9/60 = 32/60 = 8/15;

6) 7/40 + 11/70 + 13/30 = 21/120 + 52/120 + 11/70 = 73/120 + 11/70 = 511/840 + 132/840 = 643/840.