arithmetic series help Watch

Year11guy
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#1
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The sum of the first three numbers of an arithmetic series is 12. If the 20th term is -32, find the first term and the common difference.

how do i do this?
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TVIO
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Use the formula for the sum of an arithmetic series for the 3 first values. Use the formula for Un=U20 = -32. Solve simultaneously.
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Year11guy
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(Original post by TVIO)
Use the formula for the sum of an arithmetic series for the 3 first values. Use the formula for Un=U20 = -32. Solve simultaneously.
I did the formula and got 3a+3d=12. So what do I do now?
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Year11guy
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Nevermind, I've worked it out. Thanks to TVIO.
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TVIO
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(Original post by Year11guy)
I did the formula and got 3a+3d=12. So what do I do now?
You know that the 20th number is -32, so use the formula for Un and use it for U(20) = -32
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Year11guy
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(Original post by TVIO)
You know that the 20th number is -32, so use the formula for Un and use it for U(20) = -32
Thanks for the help, I've worked it out now!
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TVIO
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(Original post by Year11guy)
Thanks for the help, I've worked it out now!
Glad to help! I personally found some of those sequences questions the hardest things in the course as you have to be really careful with them.
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