Найди сумму: 1/5+2/7+1/2; 1/2+2/9+1/5; 1/4+1/3+1/5

1/5 + 2/7 + 1/2 = 69/70.

1). 1/5 + 2/7 = (1 * 7) / (5 * 7) + (2 * 5) / (7 * 5) = 7/35 + 10/35 = (7 + 10) / 35 = 17/35; 

2). 17/35 + 1/2 = (17 * 2) / (35 * 2) + (1 * 35) / (2 * 35) = 34/70 + 35/70 = 69/70.

1/2 + 2/9 + 1/5 = 83/90. 

1). 1/2 + 2/9 = (1 * 9) / (2 * 9) + (2 * 2) / (9 * 2) = 9/18 + 4/18 = 13/18; 

2). 13/18 + 1/5 = (13 * 5) / (18 * 5) + (1 * 18) / (5 * 18) = 65/90 + 18/90 = 83/90. 

1/4 + 1/3 + 1/5 = 47/70. 

1). 1/4 + 1/3 = 3/12 + 4/12 = 7/12; 

2). 7/12 + 1/5 = (7 * 5) / (12 * 5) + (1 * 12) / (5 * 12) = 35/70 + 12/70 = 47/70.