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anonymous

  • 5 years ago

help with...4x^2+8x-21=0

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  1. myininaya
    • 5 years ago
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    do you know the quadratic formula?

  2. anonymous
    • 5 years ago
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    no..i don think so?

  3. myininaya
    • 5 years ago
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    ok we can try to factor it then

  4. anonymous
    • 5 years ago
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    yeah thats what im doing..i have to factor it

  5. myininaya
    • 5 years ago
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    a=4, b=8, c=-21 Find factors of a*c that add up to be b

  6. myininaya
    • 5 years ago
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    what is a*c?

  7. anonymous
    • 5 years ago
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    a*c=-84

  8. myininaya
    • 5 years ago
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    right! a*c=-4(21) play with this until you get the sum of the factors to be b=8 -4+21 is not 8 a*c=12(-7) 12-7 is not 8 a*c=6(-14) is 6-14 =8?

  9. anonymous
    • 5 years ago
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    -8 yeah

  10. myininaya
    • 5 years ago
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    but we want 8 how can we get 8

  11. myininaya
    • 5 years ago
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    what if you said a*c=-6(14) what is the sum of those two factors?

  12. anonymous
    • 5 years ago
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    ...8!

  13. myininaya
    • 5 years ago
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    right!

  14. myininaya
    • 5 years ago
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    now replace the bx term with what you just found but you need an x behind each term so it equals bx

  15. myininaya
    • 5 years ago
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    4x^2+14x-6x-21=0

  16. anonymous
    • 5 years ago
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    ohhh okay then you have to group it right?

  17. myininaya
    • 5 years ago
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    right!

  18. anonymous
    • 5 years ago
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    so then it would be(4x^2+14x)(-6x-21)=0 which then what?

  19. myininaya
    • 5 years ago
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    no!

  20. anonymous
    • 5 years ago
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    what??

  21. myininaya
    • 5 years ago
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    what do 4x^2 and 14x have in common?____________ what do -6x and -21 have in common?___________

  22. anonymous
    • 5 years ago
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    4x ? idk

  23. myininaya
    • 5 years ago
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    2x -3 so the first 2 terms we factor out 2x(2x+7) so the second 2 terms we factor out -3(2x+7) so 4x^2+14x-6x-21=2x(2x+7)-3(2x+7) can we do anything else?

  24. myininaya
    • 5 years ago
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    you still there?

  25. myininaya
    • 5 years ago
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    we are almost there

  26. anonymous
    • 5 years ago
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    yeha ohh aha i would have never thought of that..

  27. anonymous
    • 5 years ago
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    then you do 2x-3=0 and 2x+7=0..

  28. myininaya
    • 5 years ago
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    there is 2x+7 in both of those two terms above so we can factor a 2x+7 out of both giving us (2x+7)(2x-3) this expression was orignally set =0 so we have (2x+7)(2x-3)=0 so either 2x+7=0 or 2x-3=0 when i say both its the exclusive kind

  29. myininaya
    • 5 years ago
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    right

  30. myininaya
    • 5 years ago
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    you got that part? was the factoring the problem you were having?

  31. anonymous
    • 5 years ago
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    then the answer is x=2/3 and x=2/7!!

  32. anonymous
    • 5 years ago
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    omg thanks soo much for your help!! haha sorry that took me forever to figure out:/ thanks though!

  33. myininaya
    • 5 years ago
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    x=2/3 is right but x=-7/2 for the second one

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