To add the populations given in scientific notation, we need to express both numbers with the same exponent.
The populations are:
- City A: \(3.45 \times 10^6\)
- City B: \(1.3 \times 10^5\)
First, we can convert \(1.3 \times 10^5\) to have the same exponent as \(3.45 \times 10^6\). To do this, we can express \(1.3 \times 10^5\) as \(0.13 \times 10^6\) (since moving the decimal point one place to the right decreases the exponent by 1).
Now we can add the two populations: \[ 3.45 \times 10^6 + 0.13 \times 10^6 = (3.45 + 0.13) \times 10^6 = 3.58 \times 10^6 \]
Thus, the sum of the populations of the two cities in scientific notation is: \[ \boxed{3.58 \times 10^6} \]