in an a.p the sum of first five term is equal to one fourth of the sum of the next five terms, and T20= -112.find S20
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S5 = 1/4(S10-S5)
=> substituting the value u will get :
20a + 40d = 5a + 35d
=> d = -2a --------1
=> a20 = a+(n-1)d
=> -112 = a - 38a ( from 1)
=> a = 56/19
=> s20 = 10(-2016/19)
=> s20 = 106.1 is the answer
hope it is correct and plz forgive me if wrong
=> substituting the value u will get :
20a + 40d = 5a + 35d
=> d = -2a --------1
=> a20 = a+(n-1)d
=> -112 = a - 38a ( from 1)
=> a = 56/19
=> s20 = 10(-2016/19)
=> s20 = 106.1 is the answer
hope it is correct and plz forgive me if wrong
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