Question 924391
1, -3, -7, ..., -75

a1= 1
a2= -3

d=-3-1=-4

an=-75
n=?

using the formula:
an=a1+(n-1)*d
-75=1+(n-1)*-4
-75=1-4n+4
-75=5-4n
-80=-4n
n=(-80)/(-4)
n=20 terms

1,-3,-7,-11,-15,-19,-23,-27,-31,-35,-39,-43,-47,-51,-55,-59,-63,-67,-71,-75

d=-4
1+(-4)=-3
(-3)+(-4)=-7
(-7)+(-4)=-11
(-11)+(-4)=-15
and so on...
(-71)+(-4)=-75