Show that P-7, P-10, P-13, P-16, P- 19,....... are in A.P. find its 8th term and it's common difference.    
 

T2 - T1 = P-10 - P + 7 = -3

T3 - T2 = P-13 - P + 10 = -3

=> Difference is equal

=> It is an AP

Now,

a = P-7 and d = -3

8th term = a + 7d

             = P-7 + 7(-3)

             = P - 7 - 21

             = P - 28

             

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10-7=3

common diff; d=3

also, the third term minus second term gives 3.So it is an AP.

8th term is=7+(8-1)*3=7+21=28

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