[tex]\displaystyle a).1+2+3+...+400= \frac{400(400+1)}{2} = \frac{400 \times 401}{2} = \frac{160400}{2} =80200 \\ \\ b).2+4+6+...+400=2(1+2+3+...+200)=2 \times \frac{200(200+1)}{2} = \\ \\ =2 \times \frac{200 \times 201}{2} =\not 2 \times \frac{40200}{\not 2} =40200[/tex]
