Tính: (a + b + c)^2

Tính:
a. ${(\mathrm{a} + \mathrm{b} + \mathrm{c})}^2$ ;
b. ${(\mathrm{a} + \mathrm{b} - \mathrm{c})}^2$ ;
c. ${(\mathrm{a} - \mathrm{b} - \mathrm{c})}^2$