Question

IS this 3?

Answer 1

@Vocaloid

Answer 2

hm I got something different

try writing out your an = a1 + (n-1)d formulas for the 7th term and the 10th term

Answer 3

the 7 is substituted for the d right

Answer 4

7 would be n

Answer 5

since we don't know a1 and d yet, leave those alone and substitute everything else

Answer 6

you should get one formula for a7 and one for a10

Answer 7

okay
a(7)= a1 + (7-1)d
a(10)=a1+(10-1)d

Answer 8

good, now substitute 21 for a7 and 126 for a10 since these are given

Answer 9

21 = a1 + (7-1)d
126 = a1 + (10-1)d

now subtract the two equations. you should be able to eliminate a1.

Answer 10

each separate equations right

Answer 11

21=+69(d)

Answer 12

first subtract the left hand sides

126 - 21

then subtract the right hand sides

a1 + (10-1)d - [a1 + (7-1)d]

Answer 13

let me know what you get for each side

Answer 14

3d

Answer 15

and 105

Answer 16

good and if 3d = 105, then d = ?

Answer 17

35

Answer 18

good, now we take that d value and substitute back into a7

Answer 19

21 = a1 + (7-1)(35)

a1 = ?

Answer 20

189

Answer 21

should be negative -189 ^^ but yeah that's your answer

Answer 22

haha thank you