Sequence math problem

  • Thread starter IB
  • Start date
  • #1
IB
45
0
Please help...

Given U3 = 0
S15 = -300
How can you find d? And U1?

(Instead of "T", we use "U" for "term", and d is the common difference)

Thanks!
 
Last edited:

Answers and Replies

  • #2
IB
45
0
No one can solve this problem? :(
 
  • #3
Leong
382
2
two simultaneous equations.
one equation for each information given.
solve the equations for a and d.
 
  • #4
IB
45
0
Thanks a lot!
 
  • #5
rocketboy
243
1
Set up the 2 equations:

Sn = n/2[2a + (n-1)d]
Un = a + (n-1)d

Substitute known values:

-300 = 15/2 [2a + (15-1)d]
-40 = 2a + 14d
-40 - 14d = 2a
a = -20 - 7d

0 = a + (3-1)d
0 = a + 2d
a = -2d

Set them equal to each other and solve:

-20 - 7d = -2d
-20 = 5d
d = -4

there's your d value bro...i'm sure you can do the rest! :approve:
 
  • #6
IB
45
0
Yup, my bro's so smart! :smile:
 

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