2.44. Trang 53

a) Mẫu số chung = BCNN(11, 7) = 77

Thừa số phụ: 77: 11= 7; 77:7 = 11.

Ta có:

$\dfrac{7}{11}$ + $\dfrac{5}{7}$= $\dfrac{7.7}{11.7}$ + $\dfrac{5.11}{7.11}$ = $\dfrac{49}{77}$ + $\dfrac{55}{77}$ = $\dfrac{104}{77}$

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b) Mẫu số chung = BCNN(20, 15)= 60 

Thừa số phụ: 60:20 = 3; 60:15 = 4

Ta có:

$\dfrac{7}{20}$ − $\dfrac{2}{15}$= $\dfrac{7.3}{20.3}$ − $\dfrac{2.4}{15.4}$= $\dfrac{21}{60}$ − $\dfrac{8}{60}$= $\dfrac{13}{60}$ .