If by "smaller than ten" you mean closer to zero than 10,thensum = -9 + -7 + ... + -1 + 1 + ... + 7 + 9 = 0If you mean the numbers less than 10, then:sum = 9 + 7 + ... + 1 + -1 + -3 + ... = -∞If you mean the counting numbers less than 10, then:sum = 1 + 3 + 5 + 7 + 9 = ½ × 5 × (1 + 9) = ½ × 5 × 10 = 25