x(1+1+1+5)+7=891

x8=891-7=884

x=884:8=221/2=110.5

x +  x + x + 5 * x + 2 = 891

x * 1 + x * 1 + x * 1 + 5 * x + 2 = 891

x * ( 1 + 1 + 1 + 5 ) = 891 - 2

x * 8 = 889

x =889 :8 

x = 111,125 

a,(x-5)^2=25

(x-5)^2=5^2

=>x-5=5

x=5+5=10

Vậy x=10

b,(2x+1)^2=25

(2x+1)^2=5^2

=>2x+1=5

2x=6

x=6:2

x=3

Vậy x=3

c,(3x-2^4).7^3=2.7^4

3x-2^4=2.7^4:7^3=14

3x=16+14=30

x=30:3

x=10

Vậy x=10

d,2.3^x+3^2+x=891

\(2\cdot3^x+3^{2+x}=891\\\Rightarrow 3^x\cdot2+3^x\cdot3^2=891\\\Rightarrow 3^x\cdot(2+3^2)=891\\\Rightarrow 3^x\cdot(2+9)=891\\\Rightarrow 3^x\cdot 11=891\\\Rightarrow 3^x=891:11\\\Rightarrow 3^x=81\\\Rightarrow 3^x=3^4\\\Rightarrow x=4\)

Vậy $x=4$.

\(2\cdot3^x+3^{2+x}=891\)

=>\(2\cdot3^x+3^x\cdot9=891\)

=>\(3^x=\dfrac{891}{11}=81\)

=>x=4

cho hỏi cau a sao ko co x

Co x dau ma tim

5^(2x-3) = 5^2.3 + 2.5^2

5^(2x-3) = 5^2( 3 + 2)

5^(2x-3)  = 5^2.5

5^(2x-3) = 5^3

2x - 3   =3

2x        = 6

  x         = 3

woa giỏi quá

1) (x-3)(x-5) = 0

\(\Rightarrow\orbr{\begin{cases}x-3=0\\x-5=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=3\\x=5\end{cases}}\)

(x+7).35 = 2.35

\(\Rightarrow x+7=2\)

\(\Rightarrow x=2-7=-5\)

Vậy x = -5

2) 1.2 + 2.3 + 3.4 + .... + 99.100

Đặt A = 1.2 + 2.3 + .... + 99.100

3A = 1.2.3 + 2.3.4 + 3.4.3 + .... + 99.100.3

3A = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2)+ .... + 99.100.(101-98)

3A = ( 1.2.3 + 2.3.4 + 3.4.5 +.... + 99.100.101 ) - ( 0.1.2 + 1.2.3 + 2.3.4 + ... + 98.99.100 )

3A = 99.100.101 - 0.1.2

3A = 999900 - 0

3A = 999900

A = 999900 : 3

A = 333300

Vậy A = 333300

1

\(\left(x-3\right)\left(x-5\right)=0\)

\(\Rightarrow\orbr{\begin{cases}x-3=0\\x-5=0\end{cases}\Rightarrow\orbr{\begin{cases}x=-3\\x=-5\end{cases}}}\)

\(\left(x+7\right).35=2.35\)

\(x+7=2=>x=-5\)

A=1.2+2.3+3.4...+99.100
=> 3A= 1.2.3+2.3.3+3.4.3...+99.100.3
=> 3A=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+99.100(101=98)
=> 3A= (1.2.3+2.3.4+3.4.5+...+99.100.101)-(0.1.2+1.2.3+..+98.99.200)
=> 3A= 99.100.101-0.1.2
=> 3A= 999900-0=999900
=> A= 999900:3=333300