áp dụng BĐT Cô si :
+ cho cặp số a,b ta được \(a+b\ge2\sqrt{ab}\left(1\right)\)
+ cho cặp số \(\dfrac{1}{a}+\dfrac{1}{b}\) ta được \(\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{2}{\sqrt{ab}}\left(2\right)\)
Nhân hai vế với \(\left(1\right),\left(2\right)\) ta được :\(\left(a+b\right)\left(\dfrac{1}{a}+\dfrac{1}{b}\right)\ge2\sqrt{ab}.\dfrac{2}{\sqrt{ab}}=4\) (đpcm)
Câu 3
A) ĐKXĐ : (x-3)(x+3) ≠ 0
x+3≠0
x-3≠0
B) MC : (x+3)(X-3)
A= \(\dfrac{3.\left(x-3\right)}{\left(x+3\right)\left(x-3\right)}\) + \(\dfrac{x+3}{\left(x-3\right)\left(x+3\right)}\)+\(\dfrac{18}{x^2-9}\)= \(\dfrac{3x-9+x+3+18}{MC}\)= \(\dfrac{4x+12}{MC}\) =\(\dfrac{\text{4(x+3)}}{MC}\)=\(\dfrac{4}{x-3}\)
c) Tại x=1
Có \(\dfrac{4}{1-3}\)=\(\dfrac{4}{-2}\)=-2
Làm tính chia :
a) \(\left(\dfrac{5}{7}x^2y\right)^3:\left(\dfrac{1}{7}xy\right)^3\)
b) \(\left(-x^3y^2z\right)^4:\left(-xy^2z\right)^3\)
Tính :
a) \(5x^2y^4:10x^2y\)
b) \(\dfrac{3}{4}x^3y^3:\left(-\dfrac{1}{2}x^2y^2\right)\)
c) \(\left(-xy\right)^{10}:\left(-xy\right)^5\)
Làm tính chia :
a) \(5^3:\left(-5\right)^2\)
b) \(\left(\dfrac{3}{4}\right)^5:\left(\dfrac{3}{4}\right)^3\)
c) \(\left(-12\right)^3:8^3\)
a)\(\left(a+b+c\right)^3-\left(a+b-c\right)^3-\left(b+c-a\right)^3-\left(c+a-b\right)^3\)
b)\(2a^2b^2+2b^2c^2-2c^2a^2-a^4-b^4-c^4\)
c)\(\left(a+b\right)^3+\left(b+c\right)^3+\left(c+a\right)^3-8\left(a+b+c\right)^2\)
d)\(\left(a-b\right)^5+\left(b-c\right)^5+\left(c-a\right)^5\)
Cho \(\dfrac{a}{b}=\dfrac{c}{d}\) . Chứng minh:
1. \(\dfrac{a^2+c^2}{b^2+d^2}=\dfrac{ac}{bd}\) 2. \(\dfrac{a^2+c^2}{b^2+d^2}=\dfrac{a^2-c^2}{b^2-d^2}\)
Thực hiện phép chia :
a) \(18\left(x-y\right)^{10}:2\left(x-y\right)^5\)
b) \(10\left(x-2\right)^{12}:\left(2-x\right)^{10}\)
c) \(-18\left(x-3\right)^5:2\left(3-x\right)^3\)
d) \(\left(x^2-6x+9\right):\left(x-3\right)\)
e) \(\left(x^2-x-2\right):\left(x+1\right)\)
Tính giá trị của biểu thức sau :
\(\left(-x^2y^5\right)^2:\left(-x^2y^5\right)\) tại \(x=\dfrac{1}{2};y=-1\)
Tính :
a) \(x^{10}:\left(-x\right)^8\)
b) \(\left(-x\right)^5:\left(-x\right)^3\)
c) \(\left(-y\right)^5:\left(-y\right)^4\)
