Use scientific notation to multiply the following two numbers. Express the answer using scientific notation; retain all nonzero decimal places

(1.9 • 10^4) • (8.4 • 10^5)

1.9 * 8.4 * 10^(4+5)

= 15.96 * 10^9
= 1.596 * 10^10

(8.1 . 10^8) . (4.2 . 10^3)

To multiply numbers in scientific notation, you need to multiply the two decimal parts and add the exponents.

First, multiply the decimal parts:
(1.9 * 8.4) = 15.96

Next, add the exponents:
10^4 * 10^5 = 10^(4+5) = 10^9

Now, combine the decimal part and the exponent to express the answer in scientific notation:
15.96 * 10^9

Therefore, the answer is 15.96 * 10^9.