Check whethet a,a+70,a+140,a+210,..where a=(77)^77 form an A.P or not. If yes, find the first term and common difference.

Dear Student, 

a , a+70 , a+140 , a+210 ............. 
to check whether this series is AP or not , we will have to check if the common difference is same 
common difference = a_n+1 - a_n
if n = 1 then, d = a + 70 - a = 70 [ a₂ - a₁ ]
if n = 2 then, d = a +140 - (a +70) = a +140 - a - 70 = 70  [ a₃ - a₂]
Common difference is same so this series is AP

So, the common difference = 70

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And first term of the AP is (a) which is given,  a = 77⁷⁷

Regards