The general term formula of sequence {an} is an = (- 1) ^ (n-1) * (4n-3), and the sum of the first n terms of sequence {an} can be obtained Solution: when n is even, Sn = (1-5) + (9-13) + (17-21) + ·· + [(- 1) ^ (n-2) (4n-7) + (- 1) ^ (n-1) (4n-3)] = - 4 * (n / 2) = - 2n When n is an odd number Why do you do this? I don't understand. Can you explain it? Thank you I still don't understand

The general term formula of sequence {an} is an = (- 1) ^ (n-1) * (4n-3), and the sum of the first n terms of sequence {an} can be obtained Solution: when n is even, Sn = (1-5) + (9-13) + (17-21) + ·· + [(- 1) ^ (n-2) (4n-7) + (- 1) ^ (n-1) (4n-3)] = - 4 * (n / 2) = - 2n When n is an odd number Why do you do this? I don't understand. Can you explain it? Thank you I still don't understand

When n takes odd number and even number, the symbol of the formula is different. You can remember that the n-th power problem involving - 1 in middle school mathematics generally needs to be solved in this way. Why is it just the application of classified discussion idea