Lời giải:
a.
\(\overline{a,87}+\overline{2,b2}=a+0,87+2,02+\overline{0,b}\\ =(a+\overline{0,b})+(0,87+2,02)\\ =\overline{a,b}+2,89\)
b.
\(\overline{3a,81}+\overline{4,b5}+\overline{13,9c}=30,81+a+4,05+\overline{0,b}+13,9+\overline{0,0c}\\ =(30,81+4,05+13,9)+(a+\overline{0,b}+\overline{0,0c})\\ =48,76+\overline{a,bc}=\overline{a,bc}+20,36+28,4> \overline{a,bc}+20,36+28,04 \)