a,
Bán kính (M; 4cm) là: $MA=MB= MC=4cm$
Bán kính (N; 3cm) là: $NA=NB= ND= 3cm$
Chu vi $\Delta$ MAN:$ MA+AN+MN= 13cm$
Chu vi $\Delta$ MBN: $MB+BN+MN= 13cm$
b,
$MN= MD+DN= MC-CD+DN$
$\Leftrightarrow CD= MC+DN-MN= 1cm$
c,
Các tam giác: ADM, ADO, AOC, ACN, AMO, AON, ADC, AMN, BMD, BDO, BCO, BCN, BMO, BON, BDC, BMN, MAB, NAB, DAB, CAB.
=> Có 21 tam giác.