1. cho a>0. tìm GTNN của:
a, $P=a^{2}+\frac{1}{a}$
b, $P= a+\frac{1}{a^{2}}$
2. cho $a,b,c\geqslant 0$
chứng minh răng:$a^{3}+b^{3}+c^{3}\geq a^{2}b+b^{2}c+c^{2}a$
3. cho a,b,c>0. chứng minh
$\frac{a^{3}}{b+c}+\frac{b^{3}}{c+a}+\frac{c^{3}}{a+b}\geq \frac{a^{2}+b^{2}+c^{2}}{2}$
4.cho $0^{\circ}<\alpha < 90^{\circ}$
tìm GTLL : $sin\alpha .cos^{2}\alpha$
1/ Theo AM-GM
a/ $a^{2}+\frac{1}{a}=a^{2}+\frac{1}{2a}+\frac{1}{2a}\geq 3\sqrt[3]{\frac{1}{4}}=\frac{3}{\sqrt[3]{4}}$
Dấu = khi $a=\frac{1}{\sqrt[3]{2}}$
b/ $a+\frac{1}{a^{2}}=\frac{a}{2}+\frac{a}{2}+\frac{1}{a^{2}}\geq 3.\frac{1}{\sqrt[3]{4}}$
Dấu = khi $a=\sqrt[3]{2}$
2/ Giả sử $a\geq b\geq c$
$VT-VP=\left ( a-b \right )\left ( a^{2}-b^{2} \right )+\left ( a-c \right )\left ( b^{2}-c^{2} \right )\geq 0$
Dấu = khi $a=b=c$
3/ Theo C-S
$\sum \frac{a^{3}}{b+c}=\sum \frac{a^{4}}{ab+ac}\geq \frac{\left ( \sum a^{2} \right )^{2}}{2\left ( b+bc+ca \right )}\geq \frac{\sum a^{2}}{2}$
Dấu = khi a=b=c
4/ Đặt $t=cos^{2}\alpha \geq 0$
$P^{2}=sin^{2}\alpha cos^{4}\alpha =\left ( 1-t \right )t^{2}=4\left ( 1-t \right ).\frac{t}{2}.\frac{t}{2}\leq 4.\frac{\left ( 1-t+\frac{t}{2}+\frac{t}{2} \right )^{3}}{3^{3}}=\frac{4}{27}\Rightarrow P\leq \frac{2}{3\sqrt{3}}$
Dấu = khi t=2/3
Bài viết đã được chỉnh sửa nội dung bởi Phuong Thu Quoc: 24-07-2014 - 14:34