Question

please help me it's urgent

Answer 1

Answer:

Step-by-step explanation:

its given, a=8 and d=20

sn=n/2(2a + (n-1)d)

  =n/2(2(8) + (n-1)20)

=n/2(16 + 20n -20)

=n/2(20n - 4)

Sn=n(10n-2) -----1

as When a =-30 and d=8

S2n=2n/2(2(-30) + (2n-1)8)

=n(-60 +16n -8)

S2n=n(-68 + 16n)--------(2)

It is given that sn=s2n

n(10n-2)=n(-68 + 16n)

=  10n-2=-68 + 16n

=  6n=66

=  n=11

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