???

a) \(\dfrac{81}{\left(-3\right)^n}=-243\)

\(\dfrac{\left(-3\right)^4}{\left(-3\right)^n}=\left(-3\right)^5\)

\(\left(-3\right)^n=\dfrac{\left(-3\right)^4}{\left(-3\right)^5}=\left(-3\right)^{-1}\)

n = -1

Vậy n = -1

b) \(\dfrac{25}{5^n}=5\)

\(\dfrac{5^2}{5^n}=5^1\)

\(5^n=\dfrac{5^2}{5^1}=5^1\)

n = 1

Vậy n = 1

c) \(\dfrac{1}{2}\cdot2^n+4\cdot2^n=9\cdot2^5\)

\(2^{n-1}+4\cdot2^{n-1}\cdot2=9\cdot2^5\)

\(2^{n-1}+8\cdot2^{n-1}=9\cdot2^5\)

\(\left(8+1\right)\cdot2^{n-1}=9\cdot2^5\)

\(9\cdot2^{n-1}=9\cdot2^5\)

\(2^{n-1}=2^5\cdot\dfrac{9}{9}=2^5\)

n - 1 = 5

n = 5 + 1 = 6

Vậy n = 6

a) 81/(-3)ⁿ = -243

(-3)ⁿ = 81 : (-243)

(-3)ⁿ = -1/3

n = -1

b) 25/5ⁿ = 5

5ⁿ = 25 : 5

5ⁿ = 5

n = 1

c) 1/2 . 2ⁿ + 4 . 2ⁿ = 9 . 2⁵

2ⁿ . (1/2 + 4) = 9 . 32

2ⁿ . 9/2 = 288

2ⁿ = 288 : 9/2

2ⁿ = 64

2ⁿ = 2⁶

n = 6

giúp zới

1/6-[1/6-(1/4+9/12)]
=1/6-[1/6-1]
=1/6+5/6

giúp tui zới

(-1/2+3/4)-(4/5+5/60)

= 1/4-43/60

= -7/15