① Pendulum length of a simple pendulum L=l line +D2 = 97.50 cm +2.02 cm = 98.50 cm.

The stopwatch shows: 90 seconds +9.8 seconds = 99.8 seconds.

② According to the periodic formula of simple pendulum T=2πLg.

T2=4π2Lg

According to mathematical knowledge, the slope k of T2-L image is 4 π 2g.

Then g=4π2k

So the answer is:

①98.50cm，99.8s。

②4π2k