Trong sữa chua và dưa muối có vi khuẩn lactic 

Để \(a\) là phân số

\(\dfrac{3}{n-2}\\ \Leftrightarrow n-2\ne0\\ \Leftrightarrow n\ne2\)

Vậy với \(n\ne2\) thì a là phân số 

\(a,\dfrac{5x}{2x+4}\\ ĐK:2x+4\ne0\\ \Leftrightarrow2x\ne-4\\ \Leftrightarrow x\ne-2\\ b,\dfrac{5x}{3x+9}\\ ĐK:3x+9\ne0\\ \Leftrightarrow3x\ne-9\\ \Leftrightarrow x\ne-3\)

\(A=x^2-6x+11\\ =x^2-2.3.x+3^2+2\\ =\left(x-3\right)^2+2\)

\(\left(x-3\right)^2\ge0\forall x\\ \Rightarrow\left(x-3\right)^2+2\ge2\forall x\)

Dấu = xảy ra khi

\(x-3=0\\ \Rightarrow x=3\)

Vậy \(Min_A=2\) khi \(x=3\)

\(100;110;120;130;140;150;160;170;180;190;200;....;990\)

Có : \(\left(990-100\right):10+1=90\left(số\right)\)

\(=43.\left(19-20\right)+40\\ =43.\left(-1\right)+40\\ =-43+40\\ =-3\)

\(x\left(2x^2-3\right)-x^2\left(5x+3\right)+3x^2\\ =2x^3-3x-5x^3-3x^2+3x^2\\ =\left(2x^3-5x^3\right)+\left(-3x^2+3x^2\right)-3x\\ =-3x^3+0-3x\\ =-3x^3-3x\)

\(3\left(2^2+1\right)\left(2^4+1\right)\left(2^8+1\right)\left(2^{16}+1\right)\)

\(=\left(4-1\right)\left(2^2+1\right)\left(2^4+1\right)\left(2^8+1\right)\left(2^{16}+1\right)\\ =\left(2^2-1\right)\left(2^2+1\right)\left(2^4+1\right)\left(2^8+1\right)\left(2^{16}+1\right)\\ =\left(2^4-1\right)\left(2^4+1\right)\left(2^8+1\right)\left(2^{16}+1\right)\\ =\left(2^8-1\right)\left(2^8+1\right)\left(2^{16}+1\right)\\ =\left(2^{16}-1\right)\left(2^{16}+1\right)\\ =2^{32}-1\)

Sử dụng : \(a^2-b^2=\left(a-b\right)\left(a+b\right)\)

\(300:4+300:6-25\\ =75+50-25\\ =125-25=100\)

1- I often ( read) read  books in my free time

2- What does your father (do) do every evening?

3- They ( play)are playing thế piano at the moment

4 - What is he (do) doing now?

5 -You shouldn't ( go) go out at night