Như này thì tìm Dmin nhỉ ?

D=x²-xy+x+y²-2y+2015

=>4D=4x²-4xy+4x-8y +8060

4D = (4x²+y²+1-4xy+4x-2y) +3y²-6y +8059

4D = (2x-y+1)² +3(y²-2y+1) +8056

4D=(2x-y+1)²+3(y-1)²+8056

Vì (2x-y+1)² \(\ge0\forall xy\)    3(y-1)² \(\ge0\forall y\)

=> 4D\(\ge\) 8056

=> D\(\ge\) 2014

   Dấu ''='' xảy ra khi \(\left\{{}\begin{matrix}\left(2x-y+1\right)^2=0\\3\left(y-1\right)^2=0\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}2x-y+1=0\\y-1=0\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}2x=y-1\\y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=0\\y=1\end{matrix}\right.\)

Vậy Dmin =2014 khi x=0 ;y=1

Đề yêu cầu gì bạn?

Gọi x(m) (x>0) là chiều dài mảnh đất hình chữ nhật

Chiều rộng mảnh đất hình chữ nhật là: x-6(m)

Ta có pt : x(x-6)=91

       <=>x²-6x-91=0

\(\Leftrightarrow\left\{{}\begin{matrix}x_1=13\\x_2=-7\left(loại\right)\left(x>0\right)\end{matrix}\right.\)

Vậy chiều dài là 13m 

Chiều rộng là 13-6=7 (m)

Chu vi vườn hoa là :

2(13+7)=40(m)

\(\left(\sqrt{3}-\sqrt{2}\right)\sqrt{5+2\sqrt{6}}\)

\(=\left(\sqrt{3}-\sqrt{2}\right)\sqrt{3+2+2.\sqrt{3}.\sqrt{2}}\)

\(=\left(\sqrt{3}-\sqrt{2}\right)\sqrt{\left(\sqrt{3}\right)^2+2.\sqrt{3}.\sqrt{2}+\left(\sqrt{2}\right)^2}\)

\(=\left(\sqrt{3}-\sqrt{2}\right)\sqrt{\left(\sqrt{3}+\sqrt{2}\right)^2}\)

\(=\left(\sqrt{3}-\sqrt{2}\right)\left|\sqrt{3}+\sqrt{2}\right|\)

\(=\left(\sqrt{3}-\sqrt{2}\right)\left(\sqrt{3}+\sqrt{2}\right)\)

\(=1\)

\(P=\left(\dfrac{2}{x-3}+\dfrac{x^2+3}{9-x^2}+\dfrac{x-1}{x+3}\right):\left(\dfrac{2x-1}{2x+1}-1\right)\)

\(=\left(\dfrac{2}{x-3}-\dfrac{x^2+3}{x^2-9}+\dfrac{x-1}{x+3}\right):\left(\dfrac{2x-1}{2x+1}-\dfrac{2x+1}{2x+1}\right)\)

\(=\dfrac{2\left(x+3\right)-x^2-3+\left(x-1\right)\left(x-3\right)}{\left(x-3\right)\left(x+3\right)}:\dfrac{2x-1-2x-1}{2x+1}\)

\(=\dfrac{2x+6-x^2-3+x^2-4x+3}{\left(x-3\right)\left(x+3\right)}:\dfrac{-2}{2x+1}\)

\(=\dfrac{-2x+6}{\left(x-3\right)\left(x+3\right)}:\dfrac{-2}{2x+1}\)

\(=\dfrac{-2}{\left(x+3\right)}\times\dfrac{2x+1}{-2}\)

\(=\dfrac{2x+1}{x+3}\)

`a, (2x-3)(x+5)-(x+4)(3-x)=3(x-1)(x+1)`

`<=>2x^2+10x-3x-15-3x+x^2-12+4x=3x^2-3`

`<=>8x=24`

`<=>x=3`

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`b, (4x-3)(x-1)-4x(x-2)=5`

`<=>4x^2-4x-3x+3-4x^2+8x=5`

`<=>x=2`

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`c, (x-2)(x^2+2x+4)-x(x-2)(x+3)=x^2`

`<=>x^3-8-x^3-3x^2+2x^2+6x=x^2`

`<=>2x^2-6x+8=0`

`<=>x^2-3x+4=0`

`<=>x^2-2x .3/2+9/4+7/4=0`

`<=>(x-3/2)^2=-7/4` (Vô lí)

`=>` Ptr vô nghiệm

\(a.\left(2x-3\right)\left(x+5\right)-\left(x+4\right)\left(3-x\right)=3\left(x-1\right)\left(x+1\right)\)

\(\Leftrightarrow2x^2-3x+10x-15-\left(3x-x^2+12-4x\right)=3\left(x^2-1\right)\)

\(\Leftrightarrow2x^2+7x-15-\left(-x^2-x+12\right)=3x^2-3\)

\(\Leftrightarrow2x^2+7x-15+x^2+x-12=3x^2-3\)

\(\Leftrightarrow3x^2+8x-27-3x^2+3=0\)

\(\Leftrightarrow8x-24=0\)

\(\Leftrightarrow x=3\)

\(b.\left(4x-3\right)\left(x-1\right)-4x\left(x-2\right)=5\)

\(\Leftrightarrow4x\left(x-1\right)-3\left(x-1\right)-4x.x-4x\left(-2\right)=5\)

\(\Leftrightarrow4x^2-4x-3x+3-4x^2+8x=5\)

\(\Leftrightarrow x=2\)

Gọi 3 số lẻ liên tiếp là n; n+2; n+4, theo đề bài

\(n\left(n+2\right)+n\left(n+4\right)+\left(n+2\right)\left(n+4\right)=3n\left(n+2\right)\)

\(\Leftrightarrow n\left(n+4\right)-n\left(n+2\right)+\left(n+2\right)\left(n+4\right)-n\left(n+2\right)=0\)

\(\Leftrightarrow n\left(n+4-n-2\right)+\left(n+2\right)\left(n+4-n\right)=0\)

\(\Leftrightarrow2n+4n+8=0\Leftrightarrow6n=-8\) (xem lại đề bài)

\(\left(\dfrac{7y+1}{y^2-7y}+\dfrac{7y-1}{y^2+7y}\right)\times\dfrac{y^2-49}{y^2+1}\)

(ĐKXĐ: \(y\ne0;y\ne\pm7\))

\(=\left(\dfrac{7y+1}{y.\left(y-7\right)}+\dfrac{7y-1}{y.\left(y+7\right)}\right)\times\dfrac{\left(y-7\right).\left(y+7\right)}{y^2+1}\)

\(=\dfrac{\left(7y+1\right).\left(y+7\right)+\left(7y-1\right).\left(y-7\right)}{y.\left(y-7\right).\left(y+7\right)}\times\dfrac{\left(y-7\right).\left(y+7\right)}{y^2+1}\)

\(=\dfrac{7y^2+49y+y+7+7y^2-49y-y+7}{y.\left(y-7\right).\left(y+7\right)}\times\dfrac{\left(y-7\right).\left(y+7\right)}{y^2+1}\)

\(=\dfrac{14y^2+14}{y.\left(y-7\right).\left(y+7\right)}\times\dfrac{\left(y-7\right).\left(y+7\right)}{y^2+1}\)

\(=\dfrac{14.\left(y^2+1\right)}{y.\left(y-7\right).\left(y+7\right)}\times\dfrac{\left(y-7\right).\left(y+7\right)}{y^2+1}\)

\(=\dfrac{14.\left(y^2+1\right).\left(y-7\right).\left(y+7\right)}{y.\left(y-7\right).\left(y+7\right).\left(y^2+1\right)}\)

\(=\dfrac{14}{y}\)