Question

[Nguyễn Minh Châu]

Tìm số tự nhiên x biết ;

 a )5 (x + 7 ) - 10 = 2mũ3 * 5

 b ) 9x - 2 . 3 mũ 2 = 3 mũ 4

c ) 5 mũ 25 . 5 mũ x-1 = 5 mũ 25

Answer 1

[HT.Phong (9A5)]

a) \(5\left(x+7\right)-10=2^3\cdot5\)

\(\Rightarrow5\left(x+7\right)-10=40\)

\(\Rightarrow5\left(x+7\right)=40+10\)

\(\Rightarrow x+7=\dfrac{50}{5}\)

\(\Rightarrow x+7=10\)

\(\Rightarrow x=10-7\)

\(\Rightarrow x=3\)

b) \(9x-2\cdot3^2=3^4\)

\(\Rightarrow9x-18=81\)

\(\Rightarrow9x=81+18\)

\(\Rightarrow9x=99\)

\(\Rightarrow x=\dfrac{99}{9}\)

\(\Rightarrow x=11\)

c) \(5^{25}\cdot5^{x-1}=5^{25}\)

\(\Rightarrow5^{x-1}=5^{25}:5^{25}\)

\(\Rightarrow5^{x-1}=1\)

\(\Rightarrow5^{x-1}=5^0\)

\(\Rightarrow x-1=0\)

\(\Rightarrow x=1\)

Answer 2

[Đào Trí Bình]

a) 5(�+7)−10=23⋅55(x+7)−10=23⋅5

⇒5(�+7)−10=40⇒5(x+7)−10=40

⇒5(�+7)=40+10⇒5(x+7)=40+10

⇒�+7=505⇒x+7=550​

⇒�+7=10⇒x+7=10

⇒�=10−7⇒x=10−7

⇒�=3⇒x=3

b) 9�−2⋅32=349x−2⋅32=34

⇒9�−18=81⇒9x−18=81

⇒9�=81+18⇒9x=81+18

⇒9�=99⇒9x=99

⇒�=999⇒x=999​

⇒�=11⇒x=11

c) 525⋅5�−1=525525⋅5x−1=525

⇒5�−1=525:525⇒5x−1=525:525

⇒5�−1=1⇒5x−1=1

⇒5�−1=50⇒5x−1=50

⇒�−1=0⇒x−1=0

⇒�=1⇒x=1