4.

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\(=\left(\frac{1}{9}\right)^{10}\)

Bài 4:

a/ \(\left(\frac{1}{2}\right)^{15}\cdot\left(\frac{1}{4}\right)^{20}=\left(\frac{1}{2}\right)^{15}\cdot\left[\left(\frac{1}{2}\right)^2\right]^{20}\)

\(=\left(\frac{1}{2}\right)^{15}\cdot\left(\frac{1}{2}\right)^{40}\)

\(=\left(\frac{1}{2}\right)^{55}\)

b/\(\left(\frac{1}{9}\right)^{25}\div\left(\frac{1}{3}\right)^{30}=\left(\frac{1}{9}\right)^{25}\div\left[\left(\frac{1}{3}\right)^2\right]^{15}\)

\(=\)\(\left(\frac{1}{9}\right)^{25}\div\left(\frac{1}{9}\right)^{15}\)