To determine which of the given expressions are correctly written in scientific notation, we need to understand the proper form for scientific notation. Scientific notation expresses a number as a product of a number between 1 and 10 and a power of 10. Thus, a number [tex]\(a \times 10^b\)[/tex] is in scientific notation if:
1. [tex]\(1 \leq a < 10\)[/tex]
2. [tex]\(b\)[/tex] is an integer.
Let's examine each of the given expressions:
1. [tex]\(0.71 \times 10^{-5}\)[/tex]:
- Here, the coefficient [tex]\(0.71\)[/tex] is not between 1 and 10. Hence, this is not correctly written in scientific notation.
2. [tex]\(3.28 \times 10^{-4}\)[/tex]:
- The coefficient [tex]\(3.28\)[/tex] is between 1 and 10. Therefore, this expression is correctly written in scientific notation.
3. [tex]\(16.4 \times 10^3\)[/tex]:
- The coefficient [tex]\(16.4\)[/tex] is greater than 10, so this is not correctly written in scientific notation.
4. [tex]\(6.72^2\)[/tex]:
- This expression is not in the form [tex]\(a \times 10^b\)[/tex] at all, thus it is not written in scientific notation.
Therefore, among the given options, the expression that is correctly written in scientific notation is:
[tex]\[\boxed{3.28 \times 10^{-4}}\][/tex]
