Answer :

Given, first term a = – 5


Last term tn = 45


Sum of n terms Sn = 120


To find no of terms “n”


Using Sum of n terms of an A.P. formula



where n = no. of terms


Sn = sum of n terms


Now, on substituting given value in formula we get,




120 = 20 n



To find the common difference ‘d’


We use nth term of an A.P. formula


tn = a + (n – 1)d


where n = no. of terms


a = first term


d = common difference


tn = nth terms


Thus, on substituting all values we get,


t6 = – 5 + (6 – 1)d


45 = – 5 + 5d


5d = 45 + 5 = 50



Thus, common difference is 10


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