Maths hw - arithmetic sequence

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isabellazafe
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Can u pls help me with this Q? Thanksss
Q: The first term of an arithmetic sequence is 12, the last is -78 and the sum is -363. Find the number of terms in a sequence and the difference
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davros
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(Original post by isabellazafe)
Can u pls help me with this Q? Thanksss
Q: The first term of an arithmetic sequence is 12, the last is -78 and the sum is -363. Find the number of terms in a sequence and the difference
Do you have some formulae for the nth term of a sequence and their sum?

You will need to solve some simultaneous equations
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RogerOxon
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(Original post by isabellazafe)
Can u pls help me with this Q? Thanksss
Q: The first term of an arithmetic sequence is 12, the last is -78 and the sum is -363. Find the number of terms in a sequence and the difference
You have the initial term, a=12. Can you write an expression for the nth term, in terms of a (12) and d? Set it equal to -78.

Now write an expression for the sum of n terms, setting it equal to -363.

Both of your equations are in terms of n and d. As you have two, you can solve for both n and d.
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ghostwalker
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(Original post by davros)
You will need to solve some simultaneous equations
Shouldn't be necessary if they have the formula S_n=\frac{n}{2}(a+l), though I'm not sure if it's still taught.
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Muttley79
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(Original post by ghostwalker)
Shouldn't be necessary if they have the formula S_n=\frac{n}{2}(a+l), though I'm not sure if it's still taught.
It is by my department ....
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davros
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(Original post by ghostwalker)
Shouldn't be necessary if they have the formula S_n=\frac{n}{2}(a+l), though I'm not sure if it's still taught.
That just proves I can't remember all the formulae then Even easier!
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