[tex]a)1 + 2 + 3 + ... + 60[/tex]
[tex] = \frac{60(60 + 1)}{2} = \frac{60 \times 61}{2} = 30 \times 61 = 1830[/tex]
[tex]b)1 + 2 + 3 + ... + 73[/tex]
[tex] = \frac{73(73 + 1)}{2} = \frac{73 \times 74}{2} = 73 \times 37 = 2701[/tex]
[tex]c)1 + 2 + 3 + ... + 120[/tex]
[tex] = \frac{120(120 + 1)}{2} = \frac{120 \times 121}{2} = 60 \times 121 = 7260[/tex]
[tex]d)1 + 2 + 3 + ... + 155[/tex]
[tex] = \frac{155(155 + 1)}{2} = \frac{155 \times 156}{2} = 155 \times 78 = 12090[/tex]
