Answer:
Step-by-step explanation:
9² + 10² + 11² + ......... 20² =
= 9² + 10² + 11² + 12² + 13² + 14² + 15² + 16² + 17² + 18² + 19² + 20² =
= 81 +100 +121 +144 +169 +196 +225 +256 +289 +324 +361 +400 =
= 2666
