Solution:let the geometric progression a、a+4d and a+9d.
∴q^2=(1+4d/a)^2=1+9d/a.
∴d/a=1/16,or d/a=-1/16.
When d/a=-1/16,it is not to fit this question .
So d/a=1/16.
∴ a=16d.
∵(a+q)+(a+7q)=20
∴a+4d=10
∵20d=10
∴d=1/2
∴ a=8
Solution:let the geometric progression a、a+4d and a+9d.
∴q^2=(1+4d/a)^2=1+9d/a.
∴d/a=1/16,or d/a=-1/16.
When d/a=-1/16,it is not to fit this question .
So d/a=1/16.
∴ a=16d.
∵(a+q)+(a+7q)=20
∴a+4d=10
∵20d=10
∴d=1/2
∴ a=8