Why 1 + 3 + +What about the square of (2n-5) + (2n-3) = (n-1)?
Here we use the summation formula of arithmetic sequence,
Sn (sum) = (a1 + an) n / 2, i.e. (first term + last term) × number of terms △ 2
=(1+2n-3)n/2=(n-1)²
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