Giải:

a) \(x^2+xy+y^2+1\)

\(=x^2+2.x.\dfrac{y}{2}+\left(\dfrac{y}{2}\right)^2+\dfrac{3y^2}{4}+1\)

\(=\left(x^2+2.x.\dfrac{y}{2}+\left(\dfrac{y}{2}\right)^2\right)+\dfrac{3y^2}{4}+1\)

\(=\left(x+\dfrac{y}{2}\right)^2+\dfrac{3y^2}{4}+1\ge1>0;\forall x\)

Vậy ...

Đặt \(A=x^2+5y^2+2x-4xy-10y+14\)

\(A=\left(x^2-4xy+4y^2\right)+\left(2x-4y\right)+1+y^2-6y+9+4\)

\(A=\left(x-2y\right)^2+2\left(x-2y\right)+1+\left(y-3\right)^2+4\)

\(A=\left(x-2y+1\right)^2+\left(y-3\right)^2+4\ge4>0\)

\(\Rightarrow A>0\left(đpcm\right)\)

a/ \(x^2+xy+y^2+1\)=\(\left(x^2+2x\dfrac{y}{2}+\left(\dfrac{y}{2}\right)^2\right)+\dfrac{3y^2}{4}+1\)

=\(\left(x+\dfrac{y}{2}\right)^2+\dfrac{3y^2}{4}+1\) \(\ge\)0

vậy....

b

\(a,x^2+5y^2+2x-4xy-10y+14\)

\(=x^2+2x-4xy+5y^2-10y+14\)

\(=x^2+2x\left(1-2y\right)+5y^2-10y+14\)

\(=x^2+2.x.\left(1-2y\right)+\left(1-2y\right)^2+5y^2-10y-\left(1-2y\right)^2+14\)

\(=\left(x+1-2y\right)^2+5y^2-10y-\left(1-4y+4y^2\right)+14\)

\(=\left(x+1-2y\right)^2+5y^2-10y-1+4y-4y^2+14\)

\(=\left(x+1-2y\right)^2+y^2-6y+13=\left(x+1-2y\right)^2+y^2-2.y.3+9+4\)

\(=\left(x+1-2y\right)^2+\left(y-3\right)^2+4\ge4>0\) với mọi x,y (đpcm)

b,tương tự

a/ \(x^2+xy+y^2+1=\left(x^2+xy+\frac{y^2}{4}\right)+\frac{3y^2}{4}+1=\left(x+\frac{y}{2}\right)^2+\frac{3y^2}{4}+1>0\)

b/ \(x^2+5y^2+2x-4xy-10y+14\)

\(=\left(x^2-4xy+4y^2\right)+2\left(x-2y\right)+1+\left(y^2-6y+9\right)+4\)

\(=\left(x-2y\right)^2+2\left(x-2y\right)+1+\left(y-3\right)^2+4\)

\(=\left(x-2y+1\right)^2+\left(y-3\right)^2+4>0\)

Câu hỏi của KiKyo - Toán lớp 8 - Học toán với OnlineMath

Em tham khảo nhé!

       \(x^2+4y^2+z^2-2x-6z+8y+15\)

\(=\left(x^2-2x+1\right)+\left(4y^2+8y+4\right)+\left(z^2-6z+9\right)+1\)

\(=\left(x-1\right)^2+4\left(y+1\right)^2+\left(z-3\right)^2+1>0\forall x;y\)

       \(x^2+5y^2+2x-4xy-10y+14\)

\(=\left(x^2-4xy+4y^2\right)+\left(2x-4y\right)+1+y^2-6y+9+4\)

\(=\left(x-2y\right)^2+2\left(x-2y\right)+1+\left(y-3\right)^2+4\)

\(=\left(x-2y+1\right)^2+\left(y-3\right)^2+4>0\forall x;y\)

Chúc bạn học tốt.