<PRE><B>Question 857241</B>
the formula for sum of n terms =

Sn = n/2(2a+(n-1)d)

20=n/2(2*8+(n-1)*-2)

40 = n(16-2n+2)

40=16n-2n^2+2n

40=18n-2n^2

2n^2-18n+40=0

divide by 2
n^2-9n+20=0

n^2-5n-4n+20=0

n(n-5)-4(n-5)=0

(n-5)(n-4)=0

n= 5 OR 4

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