4S=15+17+19+21+...+151+153+155

⇔4S= \(\frac{\left(15+155\right)\left[\left(155-15\right):2+1\right]}{2}\)

⇔4S=  \(\frac{170\cdot\left[140:2+1\right]}{2}\)

⇔4S=    \(\frac{170\cdot\left[70+1\right]}{2}\)

⇔4S =  \(\frac{170\cdot71}{2}\)

⇔4S= 85 * 71

⇔4S=6035

⇔S=60354

s= 60354

3.(x+4)=2.(x+15)

3x+12=2x+30

3x+12-12=2x+30-12 ( Cùng -12 ở cả 2 vế )

3x=2x+18

3x-2x=2x+18-2x ( Cùng -2x ở 2 vế )

x=18

B={101;102:103:.....}

m ko chắc là đúng hay ko 

a ) 2 . 3^x = 162

3^x = 162 : 2

3^x = 81

3^x = 3⁴

\(\Rightarrow\)x = 4

Vậy x = 4

b ) 2^x - 15 = 17

2^x = 17 + 15

2^x = 32

2^x = 2⁵

\(\Rightarrow\)x = 5

Vậy x = 5

c ) ( 7x - 11 )³ = 2⁵ . 5² + 200

( 7x - 11 )³ =  32 . 25 + 200

( 7x - 11 )³ = 800 + 200

( 7x - 11 )³ = 1000

( 7x - 11 )³ = 10³

\(\Rightarrow\)7x - 11 = 10

\(\Rightarrow\)7x = 21

\(\Rightarrow\)x = 3

Vậy x = 3

d ) 64 . 4^x = 4⁵

4³ . 4^x = 4⁵

4^x = 4⁵ : 4³

4^x = 4²

\(\Rightarrow\)x = 2

Vậy x = 2

e ) 49 .7^x = 2401

7² . 7^x = 7⁴

7^x = 7⁴ : 7²

7^x = 7²

\(\Rightarrow\)x = 2

Vậy x = 2

\(a)\)\(2.3^x=162\)

\(\Rightarrow\)\(3^x=162\div2\)

\(\Rightarrow\)\(3^x=81\)

\(\Rightarrow\)\(3^x=3^4\)

\(\Rightarrow\)\(x=4\)

\(b)\)\(2^x-15=17\)

\(\Rightarrow\)\(2^x=17+15\)

\(\Rightarrow\)\(2^x=32\)

\(\Rightarrow\)\(2^x=2^5\)

\(\Rightarrow\)\(x=5\)

\(c)\)\(\left(7x-11\right)^3=2^5.5^2+200\)

\(\Rightarrow\)\(\left(7x-11\right)^3=32.25+200\)

\(\Rightarrow\)\(\left(7x-11\right)^3=800+200\)

\(\Rightarrow\)\(\left(7x-11\right)^3=1000\)

\(\Rightarrow\)\(\left(7x-11\right)^3=10^3\)

\(\Rightarrow\)\(7x-11=10\)

\(\Rightarrow\)\(7x=10+11\)

\(\Rightarrow\)\(7x=21\)

\(\Rightarrow\)\(x=3\)

\(d)\)\(64.4^x=4^5\)

\(\Rightarrow\)\(64.4^x=1024\)

\(\Rightarrow\)\(4^x=1024\div64\)

\(\Rightarrow\)\(4^x=16\)

\(\Rightarrow\)\(4^x=4^2\)

\(\Rightarrow\)\(x=2\)

\(e)\)\(49.7^x=2401\)

\(\Rightarrow\)\(7^x=2401\div49\)

\(\Rightarrow\)\(7^x=49\)

\(\Rightarrow\)\(7^x=7^2\)

\(\Rightarrow\)\(x=2\)

B = { 1 ; 3 ; 5 ; 7 ; ... ; 19 }

Ta có :

\(\frac{98.7676-9898.76}{2001.2002}\)

\(=\frac{98.76.101-98.101.76}{2001.2002}\)

\(=\frac{0}{2001.2002}=0\)

  (98.7676 9898.76) :( 2001.2002 )

= \(\frac{98.7676-9898.76}{2001.2002}\)

= \(\frac{\left(98.76.101\right)-\left(98.101.76\right)}{2001.2002}\)

=   \(\frac{0}{2001.2002}\)

= \(0\)

5 . ( x + 2 ) + 4 . ( x + 4 ) = 125

=> 5x + 10 + 4x + 16 = 125

=> 9x + 26 = 125

=> 9x = 99

=> x = 11