What is the 15 th  term of an A.P. whose 7 th  and 11 th  terms are 51 and 79 respectively?

a+6d  =  51

a+10d= 79  (subtracting)

______________

-4d = -28

______________

=> d=7

a+6d = 51

a+(6*7) = 51

=> a= 9

therefore 15th term = a+14d

=> 9+(14*7)

=> 107

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given a+6d = 51

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