Answer :

(i) Here, we will use formula in which we can calculate sum of AP by first term(a) and last term(an) i.e.



Where n is no of terms


Clearly first term, a = 51


Last term, an = 70


And no of terms, n = 70 - 51 + 1 = 20


Hence, sum is


= 10(121)


= 1210


(ii)


Here, we will use formula in which we can calculate sum of AP by first term(a) and last term(an) i.e.



Where n is no of terms


Clearly first term,


Last term,


Common difference,


Therefore, we can calculate no of terms usinh,




⇒ 11 = n - 1


⇒ n = 12


Hence, we have sum as



⇒ Sn = 6(14) = 84


(iii)


Here, we will use formula in which we can calculate sum of AP by first term(a) and last term(an) i.e.



Where n is no of terms


Clearly first term,


Last term,


Common difference,


Therefore, we can calculate no of terms usinh,





n = 25


Hence, we have sum as





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