Calculate and give the answer as a scientific expression.

a) [[$ 9\cdot10^5 + 2\cdot10^7$]]
b) [[$ 7\cdot10^4 + 8\cdot10^6$]]
c) [[$ \text{8.2}\cdot10^{-4} - \text{1.45}\cdot10^{-5}$]]
d) [[$ \text{5.7}\cdot10^{-10} - \text{2.09}\cdot10^{-11}$]]