Four real numbers -9, A 1, A2 and-1 become arithmetic progression, so it can be seen.

1、2a 1=a2-9

2、2a2=a 1- 1

Subtract 1, a2-a 1=8/3 from Equation 2.

Five real numbers -9, b 1, b2, b2,-1 form geometric series, which shows that,

B 1 2 =-9b2，B2 2 = B 1b2，B2 2 =- 1b2，b2=- 1/9。

Then b2(a2-a 1)=-8/27.