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Introduction to completing the square.

Solve for x size 12{x} {} .

x 2 = 18 size 12{x rSup { size 8{2} } ="18"} {}

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x 2 = 0 size 12{x rSup { size 8{2} } =0} {}

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x 2 = 60 size 12{x rSup { size 8{2} } = - "60"} {}

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x 2 + 8x + 12 = 0 size 12{x rSup { size 8{2} } +8x+"12"=0} {}

  • A

    Solve by factoring.
  • B

    Now, we’re going to solve it a different way. Start by adding four to both sides.
  • C

    Now, the left side can be written as ( x + something ) 2 size 12{ \( x+"something" \) rSup { size 8{2} } } {} . Rewrite it that way, and then solve from there.
  • D

    Did you get the same answers this way that you got by factoring?
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Fill in the blanks.

( x 3 ) 2 = x 2 6x + ___ size 12{ \( x - 3 \) rSup { size 8{2} } =x rSup { size 8{2} } - 6x+"___"} {}

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( x 3 2 ) 2 = x 2 ___ x + ___ size 12{ \( x - { {3} over {2} } \) rSup { size 8{2} } =x rSup { size 8{2} } - "___"x+"___"} {}

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( x + ___ ) 2 = x 2 + 10 x + ___ size 12{ \( x+"___" \) rSup { size 8{2} } =x rSup { size 8{2} } +"10"x+"___"} {}

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( x ___ ) 2 = x 2 18 x + ___ size 12{ \( x - "___" \) rSup { size 8{2} } =x rSup { size 8{2} } - "18"x+"___"} {}

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Solve for x size 12{x} {} by completing the square.

x 2 20 x + 90 = 26 size 12{x rSup { size 8{2} } - "20"x+"90"="26"} {}

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3x 2 + 2x 4 = 0 size 12{3x rSup { size 8{2} } +2x - 4=0} {}

Start by dividing by 3. The x 2 size 12{x rSup { size 8{2} } } {} term should never have a coefficient when you are completing the square.
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Source:  OpenStax, Advanced algebra ii: activities and homework. OpenStax CNX. Sep 15, 2009 Download for free at http://cnx.org/content/col10686/1.5
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