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notsponge [240]
3 years ago
7

The second term of an arithmetic sequence is 7. The sum of the first four terms of the sequence is 12. Find the first term, a, a

nd the common difference, d, of the sequence.
Mathematics
2 answers:
dusya [7]3 years ago
8 0
T2 = 7
S4 = 12

7 = a + 6d
a = 7 - 6d

12 = 2 ( 2(7-6d) +3d )
6 = 14 - 12d + 3d
9d = 8
d = 8/9

a = 7 - 6(8/9)
a = 1/2/3
Whitepunk [10]3 years ago
3 0
Are you sure its Arithmetic?  

There isn't a way to solve this, that sum of the first four terms has got me a little screwed up.

the formula you want to use is this:

An=A1+D(N-1)

A1 is the first term of the sequence D is a common difference and N is the term of the sequence.

7= 1 + 6 (2-1) 

that's the only way, so let's plug it in for the first one 3rd and 4th 

A1= 1 + 6 (1-1)
A1=1

A3= 1 + 6 (3-1)
A3=13

A4= 1 + 6 (4-1) 

A4=19


that's as far as I can help, though.
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