Question 732799
S 100 = 100/2[2a+99d]

S100 =50[2a+99d]

a, a+d, a+4d 

(a+d)^2=a(a+4d)

a^2+2ad+d^2=a^2+4ad
d^2=2ad
d=2a

S100 =50[2a+99d]
S100= 50*100d

S100=5000d
10,000=5000d
d=2
Therefore a=1

Check

s100 = 100/2[2+198]
s100= 50*200
s100 = 10,000