Giải thích các bước giải:

a.Ta có:

$\dfrac{12-\dfrac{12}7-\dfrac{12}{289}-\dfrac{12}{85}}{4-\dfrac47-\dfrac4{289}-\dfrac4{85}}:\dfrac{3+\dfrac3{13}+\dfrac3{169}+\dfrac3{91}}{7+\dfrac7{13}+\dfrac7{169}+\dfrac7{91}}$ 

$=\dfrac{3(4-\dfrac47-\dfrac4{289}-\dfrac4{85})}{4-\dfrac47-\dfrac4{289}-\dfrac4{85}}:\dfrac{3(1+\dfrac1{13}+\dfrac1{169}+\dfrac1{91})}{7(1+\dfrac1{13}+\dfrac1{169}+\dfrac1{91})}$ 

$=3:\dfrac37$

$=3\cdot\dfrac73$

$=7$

b.Ta có:

$\dfrac{50-\dfrac4{13}+\dfrac2{15}-\dfrac2{17}}{100-\dfrac8{13}+\dfrac4{15}-\dfrac4{17}}$

$=\dfrac{50-\dfrac4{13}+\dfrac2{15}-\dfrac2{17}}{2(50-\dfrac4{13}+\dfrac2{15}-\dfrac2{17})}$

$=\dfrac12$