Question 1116627
Factor {{{ a^2-b^2-4ac+4c^2 }}}
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= {{{ (a^2-4ac+4c^2) - b^2 }}}
= {{{ (a-2c)^2 - b^2 }}}
This is the difference between two squares, and {{{ x^2-y^2 = (x-y)(x+y) }}}
= {{{ highlight( (a-2c-b)(a-2c+b) )}}}
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Check:
  {{{  (a-2c-b)(a-2c+b) =  (a^2-2ac+cross(ab))+(-2ac+4c^2 - cross(2bc))+(cross(-ab)+cross(2bc)-b^2) }}}
= {{{ a^2 - 4ac +4c^2 -b^2 }}}  (ok)