1)[latex] frac{3}{8} + frac{1}{3} = frac{9}{24} + frac{8}{24} = frac{17}{24} [/latex]2)[latex] frac{2}{5} + frac{7}{10} = 0,4+0,7=1,1[/latex]3)[latex] frac{1}{7} + frac{1}{14} + frac{1}{21} = frac{6}{42} + frac{3}{42} + frac{2}{42} = frac{11}{42} [/latex]

3/8  +  1/3 =(3 * 3  +  1*8)/24 = 9/24  +  8/24 =  17/242/5  +  7/10=(2 * 2  +  7* 1)/10 = 4 10 +  7/10 =  11/10 = 1 1/101/7  +  1/14  +  1/21 =    (1 * 6  +  1 * 3  +  1 * 2)/42 = 6/42  +3/42+2/42 =11/42