người ta mới hok có lớp 5 mà hỏi lớp 8

Thì có ai bắt người học lớp 5 trả lời đâu dangkikoduoctaobo

a/

\(9x^2+25y^2+1+30xy-6x-10y+4y^2-20y+25=0\)

\(\Leftrightarrow\left(3x+5y-1\right)^2+\left(2y-5\right)^2=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}3x+5y-1=0\\2y-5=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=-\frac{23}{6}\\y=\frac{5}{2}\end{matrix}\right.\)

b/

\(4x^2+4y^2+8xy+x^2-2x+1+y^2+2y+1=0\)

\(\Leftrightarrow4\left(x+y\right)^2+\left(x-1\right)^2+\left(y+1\right)^2=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}x+y=0\\x-1=0\\y+1=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=1\\y=-1\end{matrix}\right.\)

c/

\(y^2-2y+1+2=\frac{6}{x^2+2x+1+3}\)

\(\Leftrightarrow\left(y-1\right)^2+2=\frac{6}{\left(x+1\right)^2+3}\)

Ta có \(VT=\left(y-1\right)^2+2\ge2\)

\(\left(x+1\right)^2+3\ge3\Rightarrow VP=\frac{6}{\left(x+1\right)^2+3}\le\frac{6}{3}=2\)

\(\Rightarrow VT\ge VP\)

Dấu "=" xảy ra khi và chỉ khi: \(\left\{{}\begin{matrix}y-1=0\\x+1=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=-1\\y=1\end{matrix}\right.\)

d/

\(\frac{-9x^2+18x-9-8}{x^2-2x+1+2}=y^2+4y+4-4\)

\(\Leftrightarrow\frac{-9\left(x-1\right)^2-8}{\left(x-1\right)^2+2}=\left(y+2\right)^2-4\)

\(\Leftrightarrow\frac{-9\left(x-1\right)^2-18+10}{\left(x-1\right)^2+2}=\left(y+2\right)^2-4\)

\(\Leftrightarrow-9+\frac{10}{\left(x-1\right)^2+2}=\left(y+2\right)^2-4\)

\(\Leftrightarrow\frac{10}{\left(x-1\right)^2+2}=\left(y+2\right)^2+5\)

Ta có \(\left(x-1\right)^2+2\ge2\Rightarrow\frac{10}{\left(x-1\right)^2+2}\le\frac{10}{2}=5\Rightarrow VT\le5\)

\(\left(y+2\right)^2+5\ge5\Rightarrow VP\ge5\)

\(\Rightarrow VT\le VP\)

Dấu "=" xảy ra khi và chỉ khi: \(\left\{{}\begin{matrix}x-1=0\\y+2=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=1\\y=-2\end{matrix}\right.\)

pặc pặc....pặc pặc...........pặc pặc......

._.

Trả lời:

B = 9x² + 25y² - 30xy = ( 3x )² - 2.3x.5y + ( 5y )² = ( 3x - 5y )²

Thay x = 40; y = 4 vào B, ta có:

B = ( 3.40 - 5.4 )² = ( 120 - 20 )² = 100² = 10000

Lô chào

ta có :

\(B=9x^2+25y^2-30xy=\left(3x-5y\right)^2\)

Với x=40 và y=4 ta có : \(B=\left(3\times40-5\times4\right)^2=100^2=10000\)

phân tích thành nhân tử hả bạn?

 \(3xy-5y-6x^2+10x=\left(3xy-5y\right)-\left(6x^2-10x\right)\)

                                                 \(=y\left(3x-5\right)-2x\left(3x-5\right)\)

                                                  \(=\left(3x-5\right)\left(y-2x\right)\)

\(2x^2+30xy=5\left(x+5y\right)\sqrt{5xy}-50y^2\)\(\left(đk:x;y\ge0\right)\)

\(\Leftrightarrow2x^2+30xy-5\left(x+5y\right)\sqrt{5xy}+50y^2=0\left(1\right)\)

\(đặt:\sqrt{5xy}=b\ge0\Rightarrow5xy=b^2\Rightarrow10xy=2b^2\)

\(x+5y=a\ge0\Rightarrow x^2+10xy+25y^2=â^2\)

\(\Rightarrow2a^2=2x^2+20xy+50y^2\)

\(\Leftrightarrow\left(1\right)\Leftrightarrow2a^2+2b^2-5ab=0\Leftrightarrow\left(2a-b\right)\left(a-2b\right)=0\Leftrightarrow\left[{}\begin{matrix}b=2a\left(2\right)\\a=2b\left(3\right)\end{matrix}\right.\)

\(\left(2\right)\Rightarrow\sqrt{5xy}=2x+10y\Leftrightarrow4x^2+35xy+100y^2=0\left(4\right)\)

\(với:y=0\) \(ko\) \(là\) \(nghiệm\)

\(với:y\ne0\Rightarrow\left(4\right)\Leftrightarrow4\left(\dfrac{x}{y}\right)^2+35\left(\dfrac{x}{y}\right)+100=0\)\(\left(vô-lí\right)\)

\(do:4\left(\dfrac{x}{y}\right)^2+35\left(\dfrac{x}{y}\right)+100>0\)

\(\left(3\right)\Rightarrow x+5y=2\sqrt{5xy}\Leftrightarrow x^2+10xy+25y^2=20xy\Leftrightarrow x^2-10xy+25y^2=0\Leftrightarrow\left(x-5y\right)^2=0\Leftrightarrow x=5y\)

\(thay:x=5y\) \(vào:2x^2+y^2=51\Rightarrow2\left(5y\right)^2+y^2-51=0\Leftrightarrow51y^2-51=0\Leftrightarrow\left[{}\begin{matrix}y=1\left(tm\right)\Rightarrow x=5\left(tm\right)\\y=-1\left(loại\right)\end{matrix}\right.\)