Ngân hàng bài tập
B
Cho hàm số $f(x)=\big(1-\sqrt[4]{x}\big)\big(1+\sqrt[4]{x}\big)\big(1+\sqrt{x}\big)(1+x)$. Tính $f\left(\dfrac{1}{2^{64}}\right)$.
 | $1-\dfrac{1}{2^{128}}$ |
 | $1+\dfrac{1}{2^{64}}$ |
 | $1+\dfrac{1}{2^{128}}$ |
 | $1-\dfrac{1}{2^{64}}$ |
1 lời giải
Chọn phương án A.
$\begin{aligned}
f(x)&=\big(1-\sqrt[4]{x}\big)\big(1+\sqrt[4]{x}\big)\big(1+\sqrt{x}\big)(1+x)\\
&=\big(1-\sqrt{x}\big)\big(1+\sqrt{x}\big)(1+x)\\
&=(1-x)(1+x)\\
&=1-x^2.
\end{aligned}$
Vậy $f\left(\dfrac{1}{2^{64}}\right)=1-\dfrac{1}{2^{128}}$.