Tìm x biết a)5x^2-8x-21=0 b)4x^2-25-(5-2x)(2x+7)=0

1 bình luận về “Tìm x biết a)5x^2-8x-21=0 b)4x^2-25-(5-2x)(2x+7)=0”

1. (b)  4x^2-25-(5-2x)(2x+7)=0 

   => (4x^2 – 25) – (2x – 5)(2x + 7) = 0

   => (2x – 5)(2x + 5) – (2x – 5)(2x + 7) = 0
   

    =>(2x – 5)[(2x + 5) – (2x + 7)] = 0

   => (2x – 5)(2x + 5 – 2x – 7) = 0
   

    =>(2x – 5). (-2) = 0
   

   => 2x – 5 = 0
   => 2x = 5

   => x = 5/2

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