tìm x biết a) 2. ( x-5 ) – 2. ( x-4 ) = b + 15 . (-3) b) ( x^3 + 27 ) . ( x^2 – 25 ) = 0 c) ( x – 2 ) . ( -4 – x^2 ) bé hơn

1 bình luận về “tìm x biết a) 2. ( x-5 ) – 2. ( x-4 ) = b + 15 . (-3) b) ( x^3 + 27 ) . ( x^2 – 25 ) = 0 c) ( x – 2 ) . ( -4 – x^2 ) bé hơn”

1. a) 2.(x-5)-2(x-4)=b+15.(-3)

   $\Rightarrow$ 2x-10-2x+8=b-45

   $\Rightarrow$ -2=b+45

   $\Rightarrow$ b=-47

   Vậy b=-47

   b) (x³+27).(x²-25)=0

   $\Rightarrow$ \(\left[ \begin{array}{l}x^3+27=0\\x^2-25=0\end{array} \right.\) 

   $\Rightarrow$ \(\left[ \begin{array}{l}x^3=27\\x^2=25\end{array} \right.\) 

   $\Rightarrow$\(\left[ \begin{array}{l}x=3\\x=5 và x=-5\end{array} \right.\) 

   Vậy x=(3,5,-5)

   c)(x-2).(-4-x²) ≤0

   $\Rightarrow$ \(\left[ \begin{array}{l}x-2<0\\-4-x^2<0\end{array} \right.\) 

   $\Rightarrow$ \(\left[ \begin{array}{l}x<2\\-4<x^2\end{array} \right.\) 

   Do x²≥0 với mọi x

   $\Rightarrow$ x²>-4 với mọi x

   Vậy (x-2)(-4-x²)≥0 với mọi x

   @nguyenbidanh

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