To find the sum of the populations of the two cities, we first need to express them with the same exponent.
The populations are:
- City A: \( 3.5 \times 10^6 \)
- City B: \( 1.3 \times 10^5 \)
To add these two numbers, we can convert the population of City B to the same exponent as City A. We can do this by expressing \( 1.3 \times 10^5 \) in terms of \( 10^6 \):
\[ 1.3 \times 10^5 = 0.013 \times 10^6 \]
Now we can add the two populations:
\[ 3.5 \times 10^6 + 0.013 \times 10^6 = (3.5 + 0.013) \times 10^6 \]
Calculating the sum:
\[ 3.5 + 0.013 = 3.513 \]
Thus, the sum of the populations in scientific notation is:
\[ 3.513 \times 10^6 \]
So, the sum of the populations of the two cities is \( \mathbf{3.513 \times 10^6} \).