To go from 1^2 to 2^2 you add 3 (1+2)
To go from 2^2 to 3^2 you add 5 (2+3)
To go from 10^2 to 11^2 you add 21 (10+11)
See the pattern? You add both consecutive numbers to the square of the first to get the square of the second. In general: n^2 to (n+1)^2 you add n and n+1. To go from (n+1)^2 to (n+2)^2 add n+1 and n+2, so the increase will be 2 each time.
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