anonymous
  • anonymous
help with...4x^2+8x-21=0
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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myininaya
  • myininaya
do you know the quadratic formula?
anonymous
  • anonymous
no..i don think so?
myininaya
  • myininaya
ok we can try to factor it then

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anonymous
  • anonymous
yeah thats what im doing..i have to factor it
myininaya
  • myininaya
a=4, b=8, c=-21 Find factors of a*c that add up to be b
myininaya
  • myininaya
what is a*c?
anonymous
  • anonymous
a*c=-84
myininaya
  • myininaya
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?
anonymous
  • anonymous
-8 yeah
myininaya
  • myininaya
but we want 8 how can we get 8
myininaya
  • myininaya
what if you said a*c=-6(14) what is the sum of those two factors?
anonymous
  • anonymous
...8!
myininaya
  • myininaya
right!
myininaya
  • myininaya
now replace the bx term with what you just found but you need an x behind each term so it equals bx
myininaya
  • myininaya
4x^2+14x-6x-21=0
anonymous
  • anonymous
ohhh okay then you have to group it right?
myininaya
  • myininaya
right!
anonymous
  • anonymous
so then it would be(4x^2+14x)(-6x-21)=0 which then what?
myininaya
  • myininaya
no!
anonymous
  • anonymous
what??
myininaya
  • myininaya
what do 4x^2 and 14x have in common?____________ what do -6x and -21 have in common?___________
anonymous
  • anonymous
4x ? idk
myininaya
  • myininaya
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?
myininaya
  • myininaya
you still there?
myininaya
  • myininaya
we are almost there
anonymous
  • anonymous
yeha ohh aha i would have never thought of that..
anonymous
  • anonymous
then you do 2x-3=0 and 2x+7=0..
myininaya
  • myininaya
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
myininaya
  • myininaya
right
myininaya
  • myininaya
you got that part? was the factoring the problem you were having?
anonymous
  • anonymous
then the answer is x=2/3 and x=2/7!!
anonymous
  • anonymous
omg thanks soo much for your help!! haha sorry that took me forever to figure out:/ thanks though!
myininaya
  • myininaya
x=2/3 is right but x=-7/2 for the second one

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