N:Dure wrote:From below, what's Not the sum of consecutive numbers starting from 1?
a) 10
b) 15
c) 30
d) 45
Sum of consecutive n numbers starting from 1 = n(n + 1)/2.
=> 2*Sum = n(n + 1)
=> 2*Sum = Product of two consecutive integers
Let's check which one of the given options doesn't satisfy the above condition:
- 1. Sum = 10 => 2*Sum = 20 = 4*5 => Possible
2. Sum = 15 => 2*Sum = 30 = 5*6 => Possible
3. Sum = 30 => 2*Sum = 60 = No such product! => Not possible
4. Sum = 45 => 2*Sum = 90 = 9*10 => Possible
The correct answer is C.
