Tag Archives: Cauchy-Schwarz

Bất đẳng thức trong đề thi minh họa THPT Quốc gia môn Toán 2015

Đề bài: Cho $x$ là số thực. Tìm giá trị nhỏ nhất của $$P=\frac{\sqrt{3\left( 2{{x}^{2}}+2x+1 \right)}}{3}+\frac{1}{\sqrt{2{{x}^{2}}+\left( 3-\sqrt{3} \right)x+3}}+\frac{1}{\sqrt{2{{x}^{2}}+\left( 3+\sqrt{3} \right)x+3}}$$ Continue reading


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Bất đẳng thức chọn HSG Quốc gia, Yên Bái, 2014 – 2015

Bài toán: Cho $x,y,z$ là các số thực dương thỏa mãn $3\left ( x^{4}+y^{4}+z^{4} \right )-7\left ( x ^{2}+y^{2}+z^{2}\right )+12=0$. Tìm giá trị nhỏ nhất của biểu thức
$$P=\frac{x^{2}}{y+2z}+\frac{y^{2}}{z+2x}+\frac{z^{2}}{x+2y}.$$ Continue reading


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Bất đẳng thức trong đề thi HSG Quốc gia (VMO) 2015

Đề bài

Cho $a,b,c$ là các số thực không âm. Chứng minh rằng $$3(a^2+b^2+c^2) \ge (a+b+c)(\sqrt{ab} + \sqrt{bc} + \sqrt{ca}) + (a-b)^2+(b-c)^2+(c-a)^2 \ge (a+b+c)^2.$$ Continue reading


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Bất đẳng thức thi thử lần 1 THPT Chuyên Sư phạm Hà Nội năm 2015

Đề bài: Cho $a,b,c,d$ là các số thực dương. Chứng minh rằng $$\dfrac{(ab+cd)(ad+bc)}{(a+c)(b+d)} \ge \sqrt{abcd}.$$ Continue reading


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Đề bài: Cho các số thực không âm $a,b,c$. Chứng minh rằng :
$$\sqrt{\dfrac{a(b+c)}{a^2+bc}}+\sqrt{\dfrac{b(c+a)}{b^2+ca}}+\sqrt{\dfrac{c(a+b)}{c^2+ab}}\ge 2$$ Continue reading

Olympic 30-4 THPT chuyên Lê Quý Đôn, Bình Định

Đề bài:
Cho $x, y, z$ là ba số thực dương. Chứng minh rằng: $$\frac{1}{3}\left(\frac{yz}{x^2}+\frac{zx}{y^2}+ \frac{xy}{z^2}\right)+\left(\frac{xyz(x+y+z)}{x^2y^2+y^2z^2+z^2x^2}\right)^2 \ge 2$$ Continue reading

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Arhimede International Mathematics Competition 2008

Problem
Let $x,y$ be reals s.t. $x^2y^2\leq1$ and $n$ a natural number. Prove that:
$$ (x^n+y)^2+y^2\geq\dfrac{1}{n+2}(x^2+y^2)^n$$ Continue reading


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Bất đẳng thức thi thử lần 3 của nguoithay.vn năm 2014

Đề bài
Cho $a, \ b, \ c$ là các số thực dương thỏa mãn $a^2+b^2+c(a+b)+4c^2=4$. Tìm giá trị lớn nhất của $$P=\dfrac{a(b+c)^2}{a+c}+\dfrac{b(a+c)^2}{b+c}-\dfrac{1}{c}.$$ Continue reading