a) \(2x^2-72=0\)

\(\Leftrightarrow2\left(x^2-36\right)=0\Leftrightarrow x^2-36=0\Leftrightarrow x=_-+6\)

\(2x^2-72=0\)

\(\Leftrightarrow2x^2=72\)

\(\Leftrightarrow x^2=36\)

\(\Leftrightarrow x=\sqrt{36}=6\)

b) số xấu

\(\left[\frac{3}{4}-x\right]^3=-8\)

\(\Leftrightarrow\left[\frac{3}{4}-x\right]^3=\left(-2\right)^3\)

\(\Leftrightarrow\frac{3}{4}-x=-2\)

\(\Leftrightarrow x=\frac{3}{4}-\left(-2\right)\)

\(\Leftrightarrow x=\frac{3}{4}+2=\frac{3}{4}+\frac{8}{4}=\frac{11}{4}\)

1: x∈B(3)

=>x∈{3;6;9;12;15;18;21;...}

mà 12<=x<=18

nên x∈{12;15;18}

2: 2+4+6+...+2x=210

=>1+2+...+x=105

=>\(x\cdot\frac{\left(x+1\right)}{2}=105\)

=>x(x+1)=210

=>\(x^2+x-210=0\)

=>(x+15)(x-14)=0

=>x=-15(loại) hoặc x=14(nhận)

3: \(\left(x-2\right)^6=\left(x-2\right)^8\)

=>\(\left(x-2\right)^8-\left(x-2\right)^6=0\)

=>\(\left(x-2\right)^6\cdot\left\lbrack\left(x-2\right)^2-1\right\rbrack=0\)

=>\(\left[\begin{array}{l}\left(x-2\right)^6=0\\ \left(x-2\right)^2-1=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x-2=0\\ x-2=1\\ x-2=-1\end{array}\right.\Rightarrow\left[\begin{array}{l}x=2\\ x=3\\ x=1\end{array}\right.\)

4: 96-3(x+1)=42

=>3(x+1)=96-42=54

=>x+1=18

=>x=17

5: 15x-9x+2x=72

=>\(x\cdot\left(15-9+2\right)=72\)

=>8x=72

=>x=72/8

=>x=9

6: \(3^{x+2}+3^{x}=10\)

=>\(3^{x}\left(3^2+1\right)=10\)

=>\(3^{x}\cdot10=10\)

=>\(3^{x}=1\)

=>x=0

ai cho 2 cái để được hai điểm hỏi đáp

Bài 1:

a) A=1+2²+2⁴+.................+2¹⁰⁰

2A=(1+2²+2⁴+.................+2¹⁰⁰)

2A=2+2³+...+2¹⁰¹

2A-A=(2+2³+...+2¹⁰¹)-(1+2²+2⁴+.................+2¹⁰⁰)

A=2¹⁰¹-1

b)bạn tự làm

c) C=-1/90-1/72-1/50-1/42-1/30-1/20-1/12-1/6-1/2

\(=-\left(\frac{1}{90}+\frac{1}{72}+...+\frac{1}{2}\right)\)

\(=-\left(\frac{1}{10.9}+\frac{1}{9.8}+...+\frac{1}{2.1}\right)\)

\(=-\left(\frac{1}{10}-\frac{1}{9}+\frac{1}{9}-\frac{1}{8}+...+\frac{1}{2}-1\right)\)

\(=-\left(\frac{1}{10}-1\right)\)

\(=-\left(-\frac{9}{10}\right)=\frac{9}{10}\)

Bài 2:

cứ tính lần lượt là ra

\(2x^2-72=0\)

\(\Rightarrow2x^2=0+72\)

\(\Rightarrow2x^2=72\)

\(\Rightarrow x^2=72:2\)

\(\Rightarrow x^2=36\)

\(\Leftrightarrow x=6\)

2x²-72=0

<=> 2x²=0+72

<=> 2x²=72

<=> x²=72/2

<=> x²=36

<=> x=6 hoặc x=-6

Vậy x=6 hoặc x=-6