Question

help please !
Find the 9th term of the arithmetic sequence.
a10= 100 and a20= 50; a9=???
formula is an=a1+(n-1)d
i found the common difference where d=-5
i just need help finding a1 ...

Answer 1

d = -5 thats correct
a10 = a1 + 9d = 100

so a1 + 9(-5) = 100

solve this for a1

Answer 2

what i did was
100=a1+(10-1)(-5)
100=a1+(-45)

Answer 3

that right as far as it goes

Answer 4

so do i distribute the + to -45 ?

Answer 5

100 = a1 - 45
now add 45 to both sides

Answer 6

yes

Answer 7

145 = a1

Answer 8

yes

Answer 9

okay cool. but can you please help me go on and find the 9th term ?

Answer 10

nth term = a1 + (n-1)d
9th term = 145 + (9 - 1)* -5

Answer 11

so a9 = 105

Answer 12

thank you!

Answer 13

right - yw