the common diffrence of an ap is -2 find its sum if it\'s 1st term is 100 and last term -10

the common diffrence of an ap is -2 find its sum if it\'s 1st term is

1.	the common diffrence of an ap is -2 find its sum if it\'s 1st term is 100 and last term -10
Answer» a= 100d= -2an= -10an=a+(n-1)d-10 = 100+2-2n2n=112n=56S56=56/2(2a+(55)-2) = 28(200 - 110) = 28 x 90=2520\xa0 Given,d = 2 , a = 1 and l = 10 We have to find value of n To find the value of n the given formula is a+(n-1)d solution = a+(n-1)d = 100+(n-1)-2 = 10 (n-1)-2=10-100 = (n-1)-2= -90 = (n-1)=-90/-2(n-1) =45 = n= 45+1= n=46Now we have to find sum formula is n/2[2a+(n-1)d] 46/2 [2×100(46-1)-2] = 23(200-90) =23×110= 2530 is the sum Its sum is 2565