Dau coroana! Exercitiul 2. Va rog!

a) 2019^2 - 2019*2020 = 2019*2019 - 2019*2020= 2019*(2019-2020) = 2019×(-1) = - 2019
b) 2019^2 - 2018*2019 = 2019*(2019 - 2018) = 2019 * 1 = 2019
c) 5 + 10 + 25 +... +2020 - 5*(1+2+3+...+404) = 5*(1+2+3+...+404) - 5*(1+2+3+...+404) = 0