4a 1+4(4- 1)d/2 = 4a 1+6d & gt; = 10 the first type
S5 & lt= 15
5a 1+5(5- 1)d/2 = 5a 1+ 10d & lt; = 15 second
By the first formula -20a 1-30d < =-50.
By the second formula 20a 1+40d
10d & lt; = 10
d & lt= 1
a4=a 1+3d
a 1+4d & lt; =5
a 1+3d & lt; = five dimensions < = 4