Q. 195.0( 3 Votes )

The nth</sup

Answer :

nth term of an A.P(an) = -4n+15


a1 = -4(1)+15 = 11


a2 = -4(2)+15 = 7


d = a2- a1 = 7-11 = -4


And, Sum of first n terms of AP is given by-


Sn = (n/2)[2a+(n-1)d]


= (n/2)[2×11 + (n-1)(-4)]


= (n/2)[22-4n+4]


= (n/2)(26-4n)


= n(13-2n)


S20 = 20(13-2×20) = 20(13-40) = 20(-27) = -540


Thus, the Sum of first 20 terms of AP = -540


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