1/ -20<x<21 (x ϵ Z)

⇒ x ϵ {-19;-18;...0;1;2;...20}

⇒ (-19)+(-18)+...+0+1+2+...+20= 20

2/ -18≤x≤17 (x ϵ Z)

⇒ x ϵ {-18;-17;...0;1;2;...17}

⇒ (-18)+(-17)+...+0+1+2+...+17= -18

3/ -27<x≤27 (x ϵ Z)

⇒ x ϵ {-26;-25;...0;1;2;...27}

⇒ (-26)+(-25)+...+0+1+2+...+27= 27

4/ \(|x|\le3\) (x ϵ Z)

⇒ -3≤x≤3

⇒ x ϵ {-3;-2;-1;0;1;2;3}

⇒ (-3)+(-2)+(-1)+0+1+2+3= 0

5/ \(|-x|< 5\) (x ϵ Z)

⇒ -5<x<5

⇒ x ϵ {-4;-3;-2;-1;0;1;2;3;4}

⇒ (-4)+(-3)+(-2)+(-1)+0+1+2+3+4= 0

\(4^{2-x}=4^{16x}\)

\(2-x=16x\)

\(17x=-2\)

\(x=-\dfrac{2}{17}\)

\(9^x=18^4\)

\(3^{2x}=\left(2.9\right)^{2.2}\)

\(3^{2x}=\left(2^2.3^4\right)^2\)

\(3^{2x}=2^4.3^8\)

\(3^{2x}=2^4.3^8\)

\(3^{2x}:3^8=2^4\)

\(3^{2x-8}=2^4\)

Đến đây tôi không biết, mời các bạn giải giúp tiếp, xin cảm ơn

\(S=\dfrac{1}{1.4}+\dfrac{1}{4.7}+...+\dfrac{1}{304.307}\)

\(S=\dfrac{1}{3}\left(1-\dfrac{1}{4}\right)+\dfrac{1}{3}\left(\dfrac{1}{4}-\dfrac{1}{7}\right)+...+\dfrac{1}{3}\left(\dfrac{1}{304}-\dfrac{1}{307}\right)\)

\(S=\dfrac{1}{3}\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...-\dfrac{1}{304}+\dfrac{1}{304}-\dfrac{1}{307}\right)\)

\(S=\dfrac{1}{3}\left(1-\dfrac{1}{307}\right)\)

\(S=\dfrac{1}{3}.\dfrac{306}{307}\)

\(S=\dfrac{102}{307}\)

C) \(\left(x^2-3\right)\left(x^2+3\right)-5x^2\left(x+1\right)^2-\left(x^2-3x\right)\left(x^2-2x\right)+4x\left(x+2\right)^2\)

\(\left(x^4-9\right)-5x^2\left(x^2+2x+1\right)-\left(x^4-2x^3-3x^3+6x^2\right)+4x\left(x^2+4x+4\right)\)

\(x^4-9-5x^4-10x^3-5x^2-x^4+5x^3-6x^2+4x^3+16x^2+16x\)

\(-5x^4-x^3+5x^2+20x-9\)

D) \(-6x^2\left(x+5\right)^2-\left(x-3\right)^2+\left(x^2-2\right)\left(2x^2+1\right)-4x^2\left(3x-4\right)^2\)

\(-6x^2\left(x^2+10x+25\right)-\left(x^2-6x+9\right)+2x^4-3x^2-2-4x^2\left(9x^2-24x+16\right)\)

\(-6x^4-60x^3+150x^2-x^2+6x-9+2x^4-3x^2-2-36x^4+96x^3-64x^2\)

\(-40x^4+36x^3+82x^2+6x-11\)

Chu vi mảnh vườn là :

\(\left(5,5+3,75\right).2=18.5m\)

Số khóm hoa cần trồng là : 

\(18,5:\dfrac{1}{4}=74\left(khóm\right)\)