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Giải:
Chú ý sử dụng hằng đẳng thức :
(m+n+p)2 = m2 + n2 + p2 + 2mn + 2mp + 2np
Áp dụng hằng đẳng thức trên, ta có:
ax+by+cz = 0 ⇒ (ax+by+cz)2 = 0
⇒a2x2+b2y2+c2z2+2ax.by+2ax.cz+2by.cz=0
⇒a2x2+b2y2+c2z2= − (2abxy+2aczx+2bcyz)
Ta lại có:
bc.(y−z)2+ac.(x−z)2+ab.(x−y)2
=bc(y2−2yz+z2)+ac(x2−2xz+z2)+ab(x2−2xy+y2)
=bcy2+bcz2−2bcyz+acx2+acz2−2acxz+abx2+aby2−2abxy
=(bcy2+bcz2+acx2+acz2+abx2+aby2)−(2abxy+2aczx+2bcyz)
=bcy2+bcz2+acx2+acz2+abx2+aby2+a2x2+b2y2+c2z2
=x2(ac+ab+a2)+y2(bc+ab+b2)+z2(bc+ac+c2)
=ax2(a+b+c)+by2(a+b+c)+cz2(a+b+c)
=(a+b+c)(a.x2+b.y2+c.z2)
Vậy:
A=a.x2+b.y2+c.z2bc.(y−z)2+ac.(x−z)2+ab.(x−y)2=ax2+by2+cz2(a+b+c)(a.x2+b.y2+c.z2)
A=1a+b+cA=1a+b+c
Ta có: \(B=bc\left(y-z\right)^2+ca\left(z-x\right)^2+ab\left(x-y\right)^2\)
\(=bcy^2+bcz^2+caz^2+cax^2+aby^2-2\left(bcyz+acxz+abxy\right)\) (1)
Từ giả thiết suy ra:
\(a^2x^2+b^2y^2+c^2z^2+2\left(bcyz+acxz+abxy\right)=0\) (2)
Từ (1) và (2) suy ra:
\(B=ax^2\left(b+c\right)+by^2\left(a+c\right)+cz^2\left(a+b\right)+a^2x^2+b^2y^2+c^2z^2\)
\(=ax^2\left(a+b+c\right)+by^2\left(a+b+c\right)+cz^2\left(a+b+c\right)\)
\(=\left(a+b+c\right)\left(ax^2+by^2+cz^2\right)\)
Do đó: \(A=\dfrac{B}{ax^2+by^2+cz^2}=a+b+c\)
Đặt: B = bc(y-z)2 + ca(z-x)2 + ab(x-y)2
= bcy2 + bcz2 + caz2 + cax2 + abx2 + aby2 - 2(bcyz + acxz + abxy) (1)
=> a2x2 + b2y2 + c2z2 + 2(bcyz + acxz + abxy) = 0 (2)
Từ (1) và (2) suy ra:
B = ax2(b+c) + by2(a+c) + cz2(a+b) + a2x2 + b2y2 + c2z2
= ax2(a+b+c) + by2(a+b+c) + cz2(a+b+c)
= (az2+by2+cz2)(a+b+c)
Vậy \(A=\dfrac{B}{ax^2+by^2+cz^2}=a+b+c\)
Ta có:
\(ax+by+cz=0\Rightarrow\left(ax+by+cz\right)^2=0\)
\(\Rightarrow a^2x^2+b^2y^2+c^2z^2+2axby+2bycz+2axcz=0\)
\(\Rightarrow a^2x^2+b^2y^2+c^2z^2=-2axby-2bycz-2axcz\)
Ta có:
\(bc\left(y-z\right)^2+ca\left(z-x\right)^2+ab\left(x-y\right)^2=bc\left(y^2-2yz+z^2\right)+ca\left(z^2-2xz+x^2\right)+ab\left(x^2-2xy+y^2\right)\)
\(=bcy^2-2bcyz+bcz^2+acz^2-2acxz+acx^2+abx^2-2abxy+aby^2\)
\(=bcy^2+bcz^2+acz^2+acx^2+abx^2+aby^2-2axby-2bycz-2axcz\)
\(=bcy^2+bcz^2+acz^2+acx^2+abx^2+aby^2+a^2x^2+b^2y^2+c^2z^2\)
\(=\left(abx^2+a^2x^2+acx^2\right)+\left(bcy^2+aby^2+b^2y^2\right)+\left(bcz^2+acz^2+c^2z^2\right)\)
\(=ax^2\left(b+a+c\right)+by^2\left(c+a+b\right)+cz^2\left(b+a+c\right)\)
\(=\left(a+b+c\right)\left(ax^2+by^2+cz^2\right)\)
Thay vào A ta được:
\(A=\dfrac{\left(a+b+c\right)\left(ax^2+by^2+cz^2\right)}{ax^2+by^2+cz^2}=a+b+c\)
theo đề bài: \(ax+by+cz=0\)=> \(\left(ax+by+cz\right)^2=0\)
=> \(a^2x^2+b^2y^2+c^2z^2+2\left(axby+bycz+axcz\right)=0\left(1\right)\)
ta lại có tử số =\(bc\left(y-z\right)^2+ca\left(z-x\right)^2+ab\left(x-y\right)^2\)
=\(bcy^2+bcz^2+caz^2+acx^2+abx^2+aby^2-2\left(abxy+acxz+bcyz\right)\)(2)
từ (1)(2)=>
Tử số=\(ax^2\left(b+c\right)+by^2\left(a+c\right)+cz^2\left(a+b\right)+a^2x^2+b^2y^2+c^2z^2\)
=\(\left(ax^2+by^2+cz^2\right)\left(a+b+c\right)\)
vậy A=a+b+c
Ta có : \(ax+by+cz=0\Rightarrow\left(ax+by+cz\right)^2=0\Rightarrow a^2x^2+b^2y^2+c^2z^2+2axby+2bycz+2czax=0\Rightarrow a^2x^2+b^2y^2+c^2z^2=-2abxy-2bycz-2czax\)
Xét tử số : \(bc\left(y-z\right)^2+ac\left(z-x\right)^2+ab\left(x-y\right)^2=bc\left(y^2-2yz+z^2\right)+ac\left(z^2-2xz+x^2\right)+ab\left(x^2-2xy+y^2\right)\)\(=bcy^2+bcz^2+acx^2+acz^2+abx^2+aby^2-2\left(bcyz+abxy+acxz\right)\)
\(=bcy^2+bcz^2+acx^2+acz^2+abx^2+aby^2+a^2x^2+b^2y^2+c^2z^2\)
\(=c\left(ax^2+by^2+cz^2\right)+b\left(ax^2+by^2+cz^2\right)+a\left(ax^2+by^2+cz^2\right)\)
\(=\left(a+b+c\right)\left(ax^2+by^2+cz^2\right)\)
\(\Rightarrow A=\frac{bc\left(y-z\right)^2+ac\left(z-x\right)^2+ab\left(x-y\right)^2}{ax^2+by^2+cz^2}=\frac{\left(a+b+c\right)\left(ax^2+by^2+cz^2\right)}{ax^2+by^2+cz^2}=a+b+c\)
Giải
Ta có: \(B=bc\left(y-z\right)^2+ca\left(z-x\right)^2+ab\left(x-y\right)^2\)
\(=bcy^2+bcz^2+caz^2+cax^2+abx^2+aby^2-2\left(bcyz+acxz+abxy\right)\)
\(=ax^2\left(b+c\right)+by^2\left(a+c\right)+cz^2\left(a+b\right)-2\left(bcyz+acxz+abxy\right)\)(1)
Từ giả thiết suy ra:
\(a^2x^2+b^2y^2+c^2z^2+2\left(abxy+acxz+bcyz\right)=0\) (2)
Từ (1) và (2):
\(B=ax^2\left(b+c\right)+by^2\left(a+c\right)+cz^2\left(a+c\right)-a^2x^2-b^2y^2-c^2z^2\)
\(=ax^2\left(a+b+c\right)+by^2\left(a+b+c\right)+cz^2\left(a+b+c\right)\)
\(=\left(a+b+c\right)\left(ax^2+by^2+cz^2\right)\)
Do đó:
\(A=\frac{B}{ax^2+by^2+cz^2}=a+b+c\)
Đặt \(B=bc\left(y-z\right)^2+ca\left(z-x\right)^2+ab\left(x-y\right)^2\)
\(=bcy^2+bcz^2+caz^2+cax^2+abx^2+aby^2-2\left(bcyz+acxz+abxy\right)\)( 1 )
Mà \(a.x+by+cz=0\)
\(\Rightarrow\left(a.x+by+cz\right)^2=0^2\)
\(\Rightarrow a^2x^2+b^2y^2+c^2z^2+2\left(axby+axcz+bycz\right)=0\)( 2 )
\(\left(1\right)\left(2\right)\Rightarrow B=B+0\)
\(=bcy^2+bcz^2+caz^2+cax^2+abx^2+aby^2-2\left(bcyz+acxz+abxy\right)+a^2x^2+b^2y^2+c^2z^2+2\left(axby+axcz+bycz\right)\)
\(=a.x^2\left(b+c\right)+b.y^2\left(a+c\right)+c.z^2\left(a+b\right)+a^2x^2+b^2y^2+z^2c^2\)
\(=a.x^2\left(a+b+c\right)+b.y^2\left(a+b+c\right)+cz^2\left(a+b+c\right)\)
\(=\left(a.x^2+by^2+cz^2\right)\left(a+b+c\right)\)
\(\Rightarrow A=\frac{B}{ax^2+by^2+cz^2}=a+b+c\)
Vậy ...
\(A=\dfrac{bcy^2+bcz^2+caz^2+cax^2+abx^2+aby^2-2bcyz-2cazx-2abxy}{ax^2+by^2+cz^2}=\dfrac{\left(bcy^2+bcz^2+caz^2+cax^2+abx^2+aby^2+a^2x^2+b^2y^2+c^2z^2\right)-\left(ax+by+cz\right)^2}{ax^2+by^2+cz^2}=\dfrac{\left(ax^2+by^2+cz^2\right)\left(a+b+c\right)}{ax^2+by^2+cz^2}=a+b+c\)