find n if a=2,d=2 sn= 420
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Step-by-step explanation:
a = 2 , d = 2 , Sn = 420
Sn = n/2 {2a + (n-1)d}
420 = n/2 {4 + (n-1)2}
840 = n {4 + 2n - 2}
840 = n {2n + 2 }
2n² + 2n - 840 = 0
n² + n - 420 = 0
n² + 21n - 20n - 420 = 0
n(n+21) -20(n+21) = 0
(n-20) (n+21) = 0
n = 20 , -21
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