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a = 9
d = 17 - 9 = 8
Sn = 450
Sn = n/2 [2a + (n-1)d]
450 = n/2 [ 18 + (n-1)8 ]
450 = n/2 [2{9+(n-1)4)}]
450 = n[9 + (n-1) 4]
450 = n[9 + 4n - 4]
450 = n[5 + 4n]
450 = 5n + 4n^2
4n^2 + 5n - 450 = 0
Solving this equation using quadratic formula, we get
n = 10 or n = -11.25
Thus n = 10 (since n cant be negative so second value is rejected)
d = 17 - 9 = 8
Sn = 450
Sn = n/2 [2a + (n-1)d]
450 = n/2 [ 18 + (n-1)8 ]
450 = n/2 [2{9+(n-1)4)}]
450 = n[9 + (n-1) 4]
450 = n[9 + 4n - 4]
450 = n[5 + 4n]
450 = 5n + 4n^2
4n^2 + 5n - 450 = 0
Solving this equation using quadratic formula, we get
n = 10 or n = -11.25
Thus n = 10 (since n cant be negative so second value is rejected)
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