Question:
Which summation formula represents the series below? 1 + 2 + 6 + 24
(a) 
(b) 
(c) 
(d) 
Answer:
Option a:
is the correct answer.
Explanation:
Option a: 
By substituting the values of n and expanding the summation, we have,

Subtracting, we have,

Expanding the factorial,

Simplifying, we get,

Thus, the summation
represents the series 
Hence, Option a is the correct answer.
Option b: 
By substituting the values of n and expanding the summation, we have,

Expanding the factorial,

Simplifying, we get,

Thus, the summation
does not represents the series 
Hence, Option b is not the correct answer.
Option c: 
By substituting the values of n and expanding the summation, we have,

Adding, we have,

Expanding the factorial,

Simplifying, we get,

Thus, the summation
does not represents the series 
Hence, Option c is not the correct answer.
Option d: 
By substituting the values of n and expanding the summation, we have,

Expanding the factorial,

Simplifying, we get,

Thus, the summation
does not represents the series 
Hence, Option d is not the correct answer.
Hence, the correct answer is Option a: 