If a,b,c are in GP and a+b,b+c,c+a are in AP,then

a)b-2a=-c

b)b+3c=-2a

c)3c-b=2a

d)c+b=2a

common difference, d,

(b+c)-(a+b) = (c+a)-(b+c) = d

=> b+c-a-b = c+a-b-c

=> c-a = a-b

=> b-2a = -c

Thus it is the first option(a).

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