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a)Quy đồng hết lên:v
\(=\frac{ab\left(a-b\right)+bc\left(b-c\right)+ca\left(c-a\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}\)
\(=\frac{ab\left(a-b\right)-bc\left(a-b+c-a\right)+ca\left(c-a\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}\)
\(=\frac{\left(a-b\right)\left(ab-bc\right)+\left(c-a\right)\left(ca-bc\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}\)
\(=\frac{b\left(a-b\right)\left(a-c\right)-c\left(a-c\right)\left(a-b\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}\) (tắt xíu, ráng hiểu:v)
\(=\frac{\left(a-b\right)\left(a-c\right)\left(b-c\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}=-\frac{\left(a-b\right)\left(b-c\right)\left(c-a\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}=-1\) (đpcm)
b)(sai thì thôi, cái chỗ đẳng thức xảy ra ý) Đặt \(\frac{a}{b-c}=x;\frac{b}{c-a}=y;\frac{c}{a-b}=z\) (cho nó gọn, viết cho nó lẹ:v) theo câu a) suy ra \(xy+yz+zx=-1\) => \(2xy+2yz+2zx=-2\)
Ta cần chứng minh \(x^2+y^2+z^2\ge2\). Thêm 2xy + 2yz +2zx vào hai vế ta cần chứng minh:
\(x^2+y^2+z^2+2xy+2yz+2zx\ge2+2xy+2yz+2zx\)
\(\Leftrightarrow\left(x+y+z\right)^2\ge2-2=0\) (luôn đúng)
Ta có đpcm. Đẳng thức xảy ra khi \(x+y+z=0\)
Ta có: (a2+b2)(x2+y2)=(ax+by)2
\(\Leftrightarrow\)a2x2+a2y2+b2x2+b2y2=a2x2+2abxy+b2y2
\(\Leftrightarrow\)a2y2-2abxy+b2x2=0
\(\Leftrightarrow\)(ay-bx)2=0
\(\Leftrightarrow\)ay=bx
\(\Leftrightarrow\)\(\frac{a}{x}\)=\(\frac{b}{y}\)
#)Giải :
\(\left(a^2+b^2\right)\left(x^2+y^2\right)=\left(ax+by\right)^2\)
\(\Rightarrow a^2x^2+a^2y^2+b^2x^2+b^2y^2=a^2x^2+2abxy+b^2y^2\)
\(\Rightarrow a^2y^2+b^2x^2=2abxy\)
\(\Rightarrow a^2y^2+b^2x^2-2abxy=0\)
\(\Rightarrow\left(ay-bx\right)^2=0\)
\(\Rightarrow ay-bx=0\)
\(\Rightarrow ay=bx\)
\(\Rightarrow\frac{a}{x}=\frac{b}{y}\)(theo tính chất tỉ lệ thức)
\(\Rightarrowđpcm\)
a) Ta có: \(a^2-1\le0;b^2-1\le0;c^2-1\le0\)
\(\Rightarrow\left(a^2-1\right)\left(b^2-1\right)\left(c^2-1\right)\le0\)
\(a^2+b^2+c^2\le1+a^2b^2+b^2c^2+c^2a^2-a^2b^2c^2\le1+a^2b^2+b^2c^2+c^2a^2\) ( vì \(abc\ge0\) )
Có \(b-1\le0\Rightarrow a^2b\sqrt{b}\left(b-1\right)\le0\Rightarrow a^2b^2\le a^2b\sqrt{b}\)
Tương tự: \(\hept{\begin{cases}b^2c^2\le b^2c\sqrt{c}\\c^2a^2\le c^2a\sqrt{a}\end{cases}\Rightarrow dpcm}\)
Chi tham khao tai day:
Câu hỏi của Vương Nguyễn Thanh Triều - Toán lớp 8 - Học toán với OnlineMath
Ta có: \(\left|f\left(0\right)\right|=\left|c\right|\le k.\)
\(\left|f\left(1\right)\right|=\left|a+b+c\right|\le k\Leftrightarrow-k\le a+b+c\le k.\)(1)
\(\left|f\left(-1\right)\right|=\left|a-b+c\right|=\left|-a+b-c\right|\le k\Leftrightarrow-k\le-a+b-c\le k\).(2)
Cộng lần lượt các vế của (1) và (2) ta có: \(-2k\le2b\le2k\Leftrightarrow-k\le b\le k\Leftrightarrow\left|b\right|\le k.\)
Mặt khác ta có: \(\hept{\begin{cases}-k\le a+b+c\le k\\-k\le a-b+c\le k\end{cases}\Rightarrow-2k\le2a+2c\le2k\Leftrightarrow-k\le a+c\le k.}\)
Chọn c = k thì \(-k\le a+k\Leftrightarrow-2k\le a.\)
Chọn c = k thì \(a-k\le k\Leftrightarrow a\le2k.\) Vậy \(\left|a\right|\le2k\).
Ta có: \(\left|a\right|+\left|b\right|+\left|c\right|\le2k+k+k=4k\left(đpcm\right).\)
\(a,\left(a+b\right)\left(a^2-ab+b^2\right)+\left(a-b\right)\left(a^2+ab+b^2\right)\)\(=\left(a^3+b^3\right)+\left(a^3-b^3\right)=2a^3\Rightarrowđpcm\)
\(b,\left(a+b\right)\left[\left(a-b\right)^2+ab\right]=\left(a+b\right)\left(a^2-2ab+b^2+ab\right)=\left(a+b\right)\left(a^2-ab+b^2\right)\)\(=\left(a^3+b^3\right)\Rightarrowđpcm\)
\(c,\left(a^2+b^2\right)\left(c^2+d^2\right)=a^2c^2+a^2d^2+b^2c^2+b^2d^2=\left(a^2c^2+2abcd+b^2d^2\right)+\left(a^2d^2-2abcd+b^2c^2\right)\)\(=\left(ac+bd\right)^2+\left(ad-bc\right)^2\Rightarrowđpcm\)
a) (a+b)(a2-ab+b2)+(a-b)(a2+ab+b2)
= a3+b3+a3-b3 = 2a3
b) a3+b3
= (a+b)(a2-ab+b2)
= (a+b)(a2- 2ab+b2)+ab
= (a+b)(a2-b2)+ab
\(\text{Đặt }\dfrac{a}{x}=\dfrac{b}{y}=\dfrac{c}{z}=k \Rightarrow\left\{{}\begin{matrix}a=kx\\b=ky\\c=kz\end{matrix}\right.\\\Rightarrow\left(ax+by+cz\right)^2=\left(kx^2+ky^2+kz^2\right)^2\\ =\left(kx^2+ky^2+kz^2\right)\left(kx^2+ky^2+kz^2\right)\\ =\left(x^2+y^2+z^2\right)\left(k^2x^2+k^2y^2+k^2z^2\right) \\ =\left(x^2+y^2+z^2\right)\left(a^2+b^2+c^2\right)\left(đpcm\right)\)
\(1,VT=2\left(a^3+b^3+c^3\right)+2\left(a^2b^2+b^2c^2+c^2a^2\right)\)
Ta có \(a^3+b^3\ge ab\left(a+b\right)\)
\(b^3+c^3\ge bc\left(b+c\right)\)
\(c^3+a^3\ge ca\left(c+a\right)\)
Cộng từng vế các bđt trên ta được
\(VT\ge ab\left(a+b\right)+bc\left(b+c\right)+ca\left(c+a\right)+2\left(a^2b^2+b^2c^2+c^2a^2\right)\)
Bây giờ ta cm:
\(a^2b^2+b^2c^2+c^2a^2\ge abc\left(a+b+c\right)\)
Bất đẳng thức trên luôn đúng
Vậy bđt được chứng minh
Dấu "=" xảy ra khi a=b=c
\(\left(am+bc\right)\left(bm+ac\right)\left(cm+ab\right)\)
\(=\left[a.\left(a+b+c\right)+bc\right]\left[b.\left(a+b+c\right)+ac\right]\left[c.\left(a+b+c\right)+ab\right]\)
\(=\left(a^2+ab+ac+bc\right)\left(ba+b^2+bc+ac\right)\left(ca+cb+c^2+ab\right)\)
\(=\left[\left(a^2+ab\right)+\left(ac+bc\right)\right]\left[\left(ba+b^2\right)+\left(bc+ac\right)\right]\left[\left(ca+c^2\right)\left(cb+ab\right)\right]\)
\(=\left[a\left(a+b\right)+c\left(a+b\right)\right]\left[b\left(a+b\right)+c\left(b+a\right)\right]\left[c\left(a+c\right)b\left(b+b\right)\right]\)
\(=\left(a+b\right)\left(a+c\right)\left(a+b\right)\left(b+c\right)\left(a+c\right)\left(b+c\right)\)
\(=\left(a+b\right)^2\left(a+c\right)^2\left(b+c\right)^2\)
\(\Rightarrowđpcm\)
\(\left(am+bc\right)\left(bm+ac\right)\left(cm+ab\right)\)
\(=\left[a\left(a+b+c\right)+bc\right]\left[b\left(a+b+c\right)+ac\right]\left[c\left(a+b+c\right)+ab\right]\)
\(=\left(a^2+ab+ac+bc\right)\left(ab+b^2+bc+ac\right)\left(ac+bc+c^2+ab\right)\)
\(=\left[\left(a^2+ab\right)+\left(ac+bc\right)\right]\left[\left(ab+b^2\right)+\left(bc+ac\right)\right]\left[\left(ac+c^2\right)+\left(bc+ab\right)\right]\)
\(=\left[a\left(a+b\right)+c\left(a+b\right)\right]\left[b\left(a+b\right)+c\left(a+b\right)\right]\left[c\left(a+c\right)+b\left(a+c\right)\right]\)
\(=\left(a+c\right)\left(a+b\right)\left(b+c\right)\left(a+b\right)\left(b+c\right)\left(a+c\right)\)
\(=\left(a+b\right)^2\left(a+c\right)^2\left(b+c\right)^2\)
\(\Rightarrowđpcm\)