1 st term = 50
2 nd term = 50 - 1*9 =41
3 rd term = 50 -2*9 =32
n term = 50 -n*9
the sum of the n term = (50- 0*9) + (50 - 1*9) + (50 -2*9) +...........+[50 - (n-1)*9]
=50n - (n-1)*n*9/2 =-1450
100n-9n(n-1)=-2900
9n^2-109n -2900=0
(n-25)(9n+116)=0
n=25