Tìm x: a) ( x + 4 )^2 – ( x – 2 ).( x +2 ) = 15 b) 2 ( x – 2 )^2 – ( x + 1 )^2 = x ( x + 1 ) c) ( x + 7 ).( x – 7 ) – ( x + 3 )^2 = x – 4 d) ( 7x + 1

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1. Đáp án:

    

   Giải thích các bước giải:

   $a)(x+4)^2-(x-2)(x+2)=15$

   $⇔x^2+8x+16-x^2+4=15$

   $⇔8x=-5$

   $⇔x=-\dfrac{5}{8}$

   $b)2(x-2)^2-(x+1)^2=x(x+1)$

   $⇔2(x^2-4x+4)-(x^2+2x+1)-x^2-x=0$

   $⇔2x^2-8x+8-x^2-2x-1-x^2-x=0$

   $⇔-11x=-7$

   $⇔x=\dfrac{7}{11}$

   $c)(x+7)(x-7)-(x+3)^2=x-4$

   $⇔x^2-4x-x^2-6x-9-x=-4$

   $⇔-11x=5$

   $⇔x=-\dfrac{5}{11}$

   $d)(7x+1)^2-(7x+2)(7x-2)=-25$

   $⇔49x^2+14x+1-49x^2+4=-25$

   $⇔14x=-30$

   $⇔x=-\dfrac{15}{7}$

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