Chapter 1.1A, Problem 1A

Use the approach in Gauss’s Problem to find the following sums of arithmetic sequences ( do not use formulas):

a. $1+2+3+\mathrm{4...}+99$

b. $1+3+5+7+\mathrm{...}+1001$

c. $3+6+9+12+\mathrm{...}+300$

d. $4+8+12+16+\mathrm{...}+400$