вычислите (13/19 *3/7+13/19 *5/14+12/21*13/19 ):1/21

= 13/19(18/42+15/42+24/42):1/21=13/19(57/42):1/21==13*57*21/19*42*1=39/2=19 1/2

1) 13/19 * 3/7 = 39/1332) 13/19 * 5/14 = 65/2663) 12/21 * 13/19 = 156/3994) 39 /133 + 65/266 = 78/266 + 65 /266 = 143/2665) 143/266 + 156/399 = 429/798 + 312/798 = 741/7986) 741/798 : 1/21 = 741/798 * 21= 741/38 = 19,5