@Akai Haruma, Nguyen, Nguyễn Thị Ngọc Thơsvtkvtm

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Câu hỏi của Vũ Sơn Tùng - Toán lớp 9 | Học trực tuyến

ôi ạ, mk lm đc rồi~

Ta có: 

\(1+x^2=xy+yz+xz+x^2=\left(x+y\right)\left(x+z\right)\)

\(1+y^2=xy+yz+xz+y^2=\left(y+z\right)\left(x+y\right)\)

\(1+z^2=xy+yz+xz+z^2=\left(x+z\right)\left(y+z\right)\)

Thay vào T ta được:

\(T=x\sqrt{\frac{\left(y+z\right)\left(x+y\right)\left(x+z\right)\left(y+z\right)}{\left(x+y\right)\left(x+z\right)}}+y\sqrt{\frac{\left(x+z\right)\left(y+z\right)\left(x+y\right)\left(x+z\right)}{\left(y+z\right)\left(x+y\right)}}+z\sqrt{\frac{\left(x+y\right)\left(y+z\right)\left(x+z\right)\left(x+y\right)}{\left(x+z\right)\left(y+z\right)}}\)

\(=x\sqrt{\left(y+z\right)^2}+y\sqrt{\left(x+z\right)^2}+z\sqrt{\left(x+y\right)^2}\)

\(=x\left(y+z\right)+y\left(x+z\right)+z\left(x+y\right)\)

\(=xy+xz+xy+yz+xz+zy\)

\(=2\left(xy+yz+xz\right)=2\left(xy+yz+xz=1\right)\)

Ta có \(1+x^2=x^2+xy+yz+zx=\left(x+y\right)\left(z+x\right)\).

Tương tự ta cũng có \(1+y^2=\left(x+y\right)\left(y+z\right)\) và \(1+z^2=\left(z+x\right)\left(y+z\right)\).

Thu gọn được \(T=x\left(y+z\right)+y\left(x+z\right)+z\left(x+y\right)=2\left(xy+yz+zx\right)=2\)

Cho BT trên là S

Ta có: \(1+x^2=x^2+xy+yz+zx=\left(x+y\right)\left(x+z\right)\\ 1+y^2=\left(y+x\right)\left(y+z\right);1+z^2=\left(z+x\right)\left(z+y\right)\\ \Rightarrow S=x\left(y+z\right)+y\left(x+z\right)+z\left(x+y\right)=2\left(xy+xz+yz\right)=2\)

\(x\sqrt{\frac{\left(1+y^2\right)\left(1+z^2\right)}{1+x^2}}=x\sqrt{\frac{\left(xy+yz+xz+y^2\right)\left(xy+yz+xz+z^2\right)}{xy+yz+xz+x^2}}=x\sqrt{\frac{\left(y+z\right)\left(x+y\right)\left(y+z\right)\left(x+z\right)}{\left(x+y\right)\left(x+z\right)}}=x\sqrt{\left(y+z\right)^2}=x\left(y+z\right)\)

tương tự ta có

\(y\sqrt{\frac{\left(1+x^2\right)\left(1+z^2\right)}{1+y^2}}=y\left(x+z\right)\) và \(z\sqrt{\frac{\left(1+x^2\right)\left(1+y^2\right)}{1+z^2}}=z\left(x+y\right)\)

do đó \(A=x\left(y+z\right)+y\left(x+z\right)+z\left(x+y\right)=2\left(xy+yz+xz\right)=2.1=2\)

vậy A=2

ta có :

\(\frac{\left(1+y^2\right)\left(1+z^2\right)}{1+x^2}=\frac{\left(xy+yz+xz+y^2\right)\left(xy+yz+xz+z^2\right)}{\left(xy+yz+xz+x^2\right)}=\frac{\left(x+y\right)\left(y+z\right)\left(x+z\right)\left(y+z\right)}{\left(x+z\right)\left(x+y\right)}=\left(y+z\right)^2\)

tương tự ta sẽ có :

\(A=x\left(y+z\right)+y\left(x+z\right)+z\left(x+y\right)=2\left(xy+yz+xz\right)=2\)

1 + x² = xy + yz + zx + x² = y(x+z) + x(z+x) = (x+y).(x+z)

Tương tự, 1 + y² = (y + x). (y +z) và 1 + z² = (z +x).(z+y)

=> \(x\sqrt{\frac{\left(y+x\right)\left(y+z\right)\left(z+x\right)\left(z+y\right)}{\left(x+y\right)\left(x+z\right)}}=x\left|y+z\right|\)

Tương tự => A = x |y +z| + y.|x+ z| + z.|x+y| 

Có thể đề là rút gọn A. Yêu cầu tính A, không đủ dữ kiện ( Vid dụ : Nếu y + z > 0 và x + z< 0; x+ y < 0 => A = -2yz)

Nếu Thêm điều kiện x; y; z > 0 => A = x(y+z) + y(x+z) + z(x+y) = 2(xy + yz+ zx) = 2

\(\text{Ta có: }1+x^2=xy+yz+xz+x^2=\left(x+y\right)\left(x+z\right)\)

\(1+y^2=xy=yz=xz+y^2=\left(y+x\right)\left(y+z\right)\)

\(1+z^2=xy+yz+xz=z^2=\left(z+x\right)\left(z+y\right)\)

\(\text{Suy ra: }A=x\sqrt{\frac{\left(y+x\right)\left(y+z\right)\left(z+x\right)\left(z+y\right)}{\left(x+y\right)\left(x+z\right)}}+y\sqrt{\frac{\left(x+y\right)\left(x+z\right)\left(z+x\right)\left(z+y\right)}{\left(y+x\right)\left(y+z\right)}}\)

\(+z\sqrt{\frac{\left(y+x\right)\left(y+z\right)\left(x+y\right)\left(x+z\right)}{\left(z+x\right)\left(z+y\right)}}\)

\(=x\sqrt{\left(y+z\right)^2}+y\sqrt{\left(x+z\right)^2}+z\sqrt{\left(x+y\right)^2}\)

\(=x\left|y+z\right|+y\left|x+z\right|+z\left|x+y\right|\)

đề sai