1 + 2 + 3 + ... + 12 = ?  Choose the correct option.
A) 76	B) 68	C) 78	D) 66	E) 67

Rationale: 1 + 2 + 3 + ⋯ + n = ∑ n = n ( n + 1 ) / 2 1 + 2 + 3 + ⋯ + 12 = n ( n + 1 ) / 2 = ( 12 ( 12 + 1 ) ) / 2 = ( 12 × 13 ) / 2 = 6 × 13 = 78 answer is c
Answer: C