Ta có:

a = $\frac{35}{49}=\frac{5}{7};$

b = $\sqrt{\frac{{{5}^{2}}}{{{7}^{2}}}}=\frac{5}{7}$;       

c = $\frac{\sqrt{{{5}^{2}}}+\sqrt{{{35}^{2}}}}{\sqrt{{{7}^{2}}}+\sqrt{{{49}^{2}}}}=\frac{5+35}{7+49}=\frac{40}{56}=\frac{5}{7}$;

d = $\frac{\sqrt{{{5}^{2}}}-\sqrt{{{35}^{2}}}}{\sqrt{{{7}^{2}}}-\sqrt{{{49}^{2}}}}=\frac{5-35}{7-49}=\frac{-30}{-42}=\frac{5}{7}$

Vậy a = b = c = d.