Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
\(B=\left(\dfrac{a-b}{a^2+ab}-\dfrac{a}{b^2+ab}\right):\left(\dfrac{b^3}{a^3-ab^2}+\dfrac{1}{a+b}\right)\)
\(=\left(\dfrac{a-b}{a\left(a+b\right)}-\dfrac{a}{b\left(a+b\right)}\right):\left(\dfrac{b^3}{a\left(a-b\right)\left(a+b\right)}+\dfrac{1}{a+b}\right)\)
\(=\dfrac{b\left(a-b\right)-a^2}{ab\left(a+b\right)}:\dfrac{b^3+a\left(a-b\right)}{a\left(a-b\right)\left(a+b\right)}\)
\(=\dfrac{ab-b^2-a^2}{ab\left(a+b\right)}\cdot\dfrac{a\left(a-b\right)\left(a+b\right)}{a^2-ab+b^3}\)
\(=\dfrac{\left(a-b\right)\left(ab-b^2-a^2\right)}{b\left(a^2-ab+b^3\right)}\)
\(=\dfrac{-\left(a-b\right)\left(a^2-ab+b^2\right)}{b\left(a^2-ab+b^3\right)}\)
Đề lỗi rồi chứ mình ko rút gọn đc nữa
Lời giải:
Xét:
\(\frac{a^4}{(a+b)(a^2+b^2)}+\frac{b^4}{(b+c)(b^2+c^2)}+\frac{c^4}{(c+a)(c^2+a^2)}-\left[\frac{b^4}{(a+b)(a^2+b^2}+\frac{c^4}{(b+c)(b^+c^2)}+\frac{a^4}{(c+a)(c^2+a^2)}\right]\)
\(=\frac{a^4-b^4}{(a+b)(a^2+b^2)}+\frac{b^4-c^4}{(b+c)(b^2+c^2)}+\frac{c^4-a^4}{(c+a)(c^2+a^2)}=a-b+b-c+c-a=0\)
\(\Rightarrow \frac{a^4}{(a+b)(a^2+b^2)}+\frac{b^4}{(b+c)(b^2+c^2)}+\frac{c^4}{(c+a)(c^2+a^2)}=\frac{b^4}{(a+b)(a^2+b^2}+\frac{c^4}{(b+c)(b^+c^2)}+\frac{a^4}{(c+a)(c^2+a^2)}\)
\(\Rightarrow 2P=\frac{a^4+b^4}{(a+b)(a^2+b^2)}+\frac{b^4+c^4}{(b+c)(b^2+c^2)}+\frac{c^4+a^4}{(c+a)(c^2+a^2)}\)
Áp dụng hệ quả quen thuộc của BĐT AM-GM: \(x^2+y^2\geq \frac{(x+y)^2}{2}\) ta có:
\(a^4+b^4\geq \frac{(a^2+b^2)^2}{2}\)
\(a^2+b^2\geq \frac{(a+b)^2}{2}\)
\(\Rightarrow a^4+b^4\geq \frac{(a^2+b^2).\frac{(a+b)^2}{2}}{2}=\frac{(a^2+b^2)(a+b)^2}{4}\)
\(\Rightarrow \frac{a^4+b^4}{(a+b)(a^2+b^2)}\geq \frac{a+b}{4}\). Tương tự với các phân thức còn lại:
\(\Rightarrow 2P\geq \frac{a+b}{4}+\frac{b+c}{4}+\frac{c+a}{4}=\frac{a+b+c}{2}=2\)
\(\Rightarrow P\geq 1\). Vậy \(P_{\min}=1\Leftrightarrow a=b=c=\frac{4}{3}\)
\(a=b=c=\dfrac{4}{3}\Rightarrow P=1\)
Ta se cm \(P=1\) la GTNN cua P hay \(Σ\dfrac{a^4}{\left(a+b\right)\left(a^2+b^2\right)}\ge1\)
C-S: \(VT\ge\dfrac{\left(a^2+b^2+c^2\right)^2}{Σ\left(a+b\right)\left(a^2+b^2\right)}\)
Hay ta can cm bdt \(\dfrac{\left(a^2+b^2+c^2\right)^2}{Σ\left(a+b\right)\left(a^2+b^2\right)}\ge1=\dfrac{a+b+c}{4}\)
\(\Leftrightarrow4\left(a^2+b^2+c^2\right)^2\ge\left(a+b+c\right)\left(Σ\left(a+b\right)\left(a^2+b^2\right)\right)\)
\(\LeftrightarrowΣ\left(a-b\right)^2\left(a^2+b^2+c^2-ab\right)\ge0\)
\(BT=\frac{a^2\left(b-c\right)+b^2c-b^2a+c^2a-c^2b}{a^4\left(b^2-c^2\right)+b^4c^2-b^4a^2+c^4a^2-c^4b^2}\)
\(=\frac{a^2\left(b-c\right)+bc\left(b-c\right)-a\left(b^2-c^2\right)}{a^4\left(b^2-c^2\right)+b^2c^2\left(b^2-c^2\right)-\left(b^4-c^4\right)a^2}\)
\(=\frac{\left(b-c\right)\left(a^2+bc-a\left(b+c\right)\right)}{\left(b^2-c^2\right)\left(a^4+b^2c^2-a^2\left(b^2+c^2\right)\right)}\)
\(=\frac{\left(a-b\right)\left(a-c\right)}{\left(b+c\right)\left(a^2-b^2\right)\left(a^2-c^2\right)}\)
\(=\frac{1}{\left(a+b\right)\left(b+c\right)\left(a+c\right)}\)
\(\frac{a^2\left(b-c\right)+b^2\left(c-a\right)+c^2\left(a-b\right)}{a^4\left(b^2-c^2\right)+b^4\left(c^2-a^2\right)+c^4\left(a^2-b^2\right)}\)
= \(\frac{a^2\left(b-c\right)+b^2c-c^2b-a\left(b^2-c^2\right)}{a^4\left(b^2-c^2\right)+b^4c^2-c^4b^2-a^2\left(a^4-b^4\right)}\)
= \(\frac{\left(b-c\right)\left(a-b\right)\left(c-a\right)}{\left(b^2-c^2\right)\left(a^2-b^2\right)\left(c^2-a^2\right)}\)
= \(\frac{1}{\left(b+c\right)\left(a+b\right)\left(c+a\right)}\)