Question

Tìm x thuộc N biết rằng:

a/\(8< 2^x\)bé bằng\(2^6.2^{-2}\)

b/\(\left(\dfrac{2}{5}\right)^x>\left(\dfrac{5}{2}\right)^{-3}.\left(-\dfrac{2}{5}\right)^2\)

Answer 1

a: \(\Leftrightarrow2^3< 2^x< 2^4\)

=>3<x<4

hay \(x\in\varnothing\)

b: \(\Leftrightarrow\left(\dfrac{2}{5}\right)^x>\left(\dfrac{2}{5}\right)^3\cdot\left(\dfrac{2}{5}\right)^2=\left(\dfrac{2}{5}\right)^5\)

=>x>5