\(C=\dfrac{-1}{\left(x-6\sqrt{x}+9\right)+1}=\dfrac{-1}{\left(\sqrt{x}-3\right)^2+1}\ge\dfrac{-1}{1}=-1\)

\(C_{min}=-1\) khi \(x=9\)

\(E=\dfrac{2}{-4-\left(x-2\sqrt{x}+1\right)}=\dfrac{2}{-4-\left(\sqrt{x}-1\right)^2}\ge\dfrac{2}{-4}=-\dfrac{1}{2}\)

\(E_{min}=-\dfrac{1}{2}\) khi \(x=1\)

\(B=\dfrac{2}{-1-\left(x-4\sqrt{x}+4\right)}=\dfrac{2}{-1-\left(\sqrt{x}-2\right)^2}\ge\dfrac{2}{-1}=-2\)

\(B_{min}=-2\) khi \(x=4\)

\(D=\dfrac{-3}{x+8\sqrt{x}+20}\ge-\dfrac{3}{20}\)

\(D_{min}=-\dfrac{3}{20}\) khi \(x=0\)

\(A=\dfrac{2}{4\sqrt{x}-x-5}=\dfrac{2}{-\left(x-4\sqrt{x}+4+1\right)}=\dfrac{2}{-\left(\sqrt{x}-2\right)^2-1}\)

\(-\left(\sqrt{x}-2\right)^2-1\le-1\\ \Leftrightarrow\dfrac{2}{-\left(\sqrt{x}-2\right)^2-1}\ge-2\)

Dấu \("="\Leftrightarrow x=4\)

\(B=\dfrac{-3}{x+8\sqrt{x}+20}=\dfrac{-3}{\left(\sqrt{x}+4\right)^2+4}\)

\(\left(\sqrt{x}+4\right)^2+4\ge4\\ \Leftrightarrow\dfrac{3}{\left(\sqrt{x}+4\right)^2+4}\le\dfrac{3}{4}\\ \Leftrightarrow\dfrac{-3}{\left(\sqrt{x}+4\right)^2+4}\ge-\dfrac{3}{4}\)

Dấu \("="\Leftrightarrow\sqrt{x}+4=0\Leftrightarrow x\in\varnothing\)

Vậy dấu \("="\) không xảy ra

Bài 1:

a. \(Q=A=UIt=220\left(\dfrac{1000}{220}\right)\cdot1=1000\left(J\right)\)

b. \(A'=Pt=1000\cdot3\cdot30=90000\)Wh = 90kWh

\(\Rightarrow T=A'\cdot1000=90\cdot1000=90000\left(dong\right)\)

Bài 1.

Điện trở bếp: \(R_b=\dfrac{U_b^2}{P_b}=\dfrac{220^2}{1000}=48,4\Omega\)

Dòng điện qua bếp: \(I_b=\dfrac{P_b}{U_b}=\dfrac{1000}{220}=\dfrac{50}{11}A\)

Nhiệt lượng bếp tỏa ra trong 1s:

\(Q=RI^2=48,4\cdot\left(\dfrac{50}{11}\right)^2\cdot1=1000J\)

Điện năng tiêu thụ của bếp trong 1 tháng:

 \(A=UIt=220\cdot\dfrac{50}{11}\cdot30\cdot3\cdot3600=324000000J=90kWh\)

Tiền phải trả:

 \(T=90\cdot1000=90000\left(đồng\right)\)

Bài 2:

a. \(I=P:U=900:220=\dfrac{45}{11}A\)

b. \(R=U:I=220:\dfrac{45}{11}\approx53,8\Omega\)

c. \(A=Pt=900\cdot\dfrac{20}{60}=300\)Wh = 0,3kWh

d. \(T=A\cdot1500=0,3\cdot1500\cdot30=13500\left(dong\right)\)

4\(\times\)7\(\times\)25\(\times\)3

= (4 \(\times\) 25) \(\times\) (7 \(\times\) 3)

= 100 \(\times\) 21

= 2 100 

=4*7*25*3

=[3*7]*[4*25]

=21*100

2100