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