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Math Help - Formula for Arithmetic sequence

  1. #1
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    Formula for Arithmetic sequence

    find the formula for an for the arithmetic sequence.

    a3=94, a6=85



    ok i know how to solve a similar problem when i am given a1 and d. but i dont have either A1 or d so how can i find them? to plug into an=dn+c and c= a1-d?? please help me understand this thank you.
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  2. #2
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    Quote Originally Posted by flexus View Post
    find the formula for an for the arithmetic sequence.

    a3=94, a6=85



    ok i know how to solve a similar problem when i am given a1 and d. but i dont have either A1 or d so how can i find them? to plug into an=dn+c and c= a1-d?? please help me understand this thank you.

    HI

    a+2d=94 --- 1

    a+5d=85 --- 2

    Solve for a and d then plug into the general equation of Ap
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  3. #3
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    Quote Originally Posted by mathaddict View Post
    HI

    a+2d=94 --- 1

    a+5d=85 --- 2

    Solve for a and d then plug into the general equation of Ap
    i don't understand where you got that from. could you explain to me what this all means. im sorry i am just a little behind on all this stuff. im looking for A1 and d.
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  4. #4
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    Quote Originally Posted by flexus View Post
    i don't understand where you got that from. could you explain to me what this all means. im sorry i am just a little behind on all this stuff. im looking for A1 and d.
    Well , look at the general form for AP

    T_n=a+(n-1)d

    So T_2=a+(2-1)d\Rightarrow a+d

     <br />
T_6=a+(6-1)d\Rightarrow a+5d<br />
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  5. #5
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    Quote Originally Posted by mathaddict View Post
    HI

    a+2d=94 --- 1

    a+5d=85 --- 2

    Solve for a and d then plug into the general equation of Ap

    i don't know what you did but it made no sense to me. i actually figured it out. what i did was

    a6-a3= 3 terms apart

    a6=a3+3d--> 85=94+3d--> d=-3

    an=a1+(n-1)d
    94=a1+(3-1)-3-->a1=100

    c=a1-d-->c=100-(-3)-->c=103

    an=-3n+103=answer!!!!!!!

    thank you very much for your time and help mathaddict! it was greatly Appreciated.
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  6. #6
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    Quote Originally Posted by flexus View Post
    i don't know what you did but it made no sense to me. i actually figured it out. what i did was

    a6-a3= 3 terms apart

    a6=a3+3d--> 85=94+3d--> d=-3

    an=a1+(n-1)d
    94=a1+(3-1)-3-->a1=100

    c=a1-d-->c=100-(-3)-->c=103

    an=-3n+103=answer!!!!!!!
    OK , your method works fine too .. Then you stick to your method to make things easier .
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