8. Given the inner diameter of a ring is $a \mathrm{~cm}$, and the outer diameter is $b \mathrm{~cm}$, 50 such rings are linked together one by one to form a chain. Then, the length of the chain when stretched out is $\qquad$ $\mathrm{cm}$.
$8.49 a+b$.
As shown in the figure, when there are 3 rings, the chain length is
$$
3 a+\frac{b-a}{2} \times 2=2 a+b(\mathrm{~cm}) \text {. }
$$

When there are 50 rings, the chain length is
$$
50 a+2 \times \frac{b-a}{2}=49 a+b(\mathrm{~cm}) .
$$