Finding d and U1 from U3 and S15

[IB]

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!

 

[IB]

No one can solve this problem? :(

 

[Leong]

two simultaneous equations.
one equation for each information given.
solve the equations for a and d.

 

[IB]

Thanks a lot!

 

[rocketboy]

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!

 

[IB]

Yup, my bro's so smart!