[tex]\displaystyle a).4+8+12+...+360=4(1+2+3+...+90)=4 \times \frac{90(90+1)}{2} = \\ \\ =4 \times \frac{90 \times 91}{2} =4 \times \frac{8190}{2} =4 \times 4095=16380 \\ \\ b).5+10+15+...+625=5(1+1+3+...+125)= \\ \\ =5 \times \frac{125(125+1)}{2} =5 \times \frac{125 \times 126}{2} =5 \times \frac{15750}{2} =5 \times 7875=39375[/tex]
