Shortcut Trick
Each pair of terms (1 − 2), (3 − 4), (5 − 6) results in −1.
Total number of terms = 100.
Total number of pairs = 100 ÷ 2 = 50 pairs.
Value of expression = 50 × (−1) = −50.
∴ The correct answer is -50.
Alternate Method
Given: 1 − 2 + 3 − 4 + 5 − 6 ..... to 100 terms.
Formula Used: Sum of Arithmetic Progression (AP) = (n÷2)[2a + (n−1)d]
Calculations:
Separate the positive and negative terms into two series:
⇒ Sum = (1 + 3 + 5 + ... to 50 terms) − (2 + 4 + 6 + ... to 50 terms)
⇒ Positive series: n = 50, a = 1, d = 2
⇒ Sumpos = (50÷2)[2(1) + (50−1)2] = 25[2 + 98] = 25 × 100 = 2500
⇒ Negative series: n = 50, a = 2, d = 2
⇒ Sumneg = (50÷2)[2(2) + (50−1)2] = 25[4 + 98] = 25 × 102 = 2550
⇒ Total Sum = 2500 − 2550 = −50
∴ The correct answer is -50.
Additional Information
Sum of First n Odd Numbers
The sum of the first n odd natural numbers is always given by the formula n2.
Sum of First n Even Numbers
The sum of the first n even natural numbers is given by the formula n(n + 1).
Alternating Sum of first n terms
For an alternating series 1 − 2 + 3 − 4 ..., if n is even, the sum is −n/2. If n is odd, the sum is (n+1)/2.