Question

find n if a=2,d=2 sn= 420​

Answer 1

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