987,654,321^2 = 975,461,057,789,971,041

Work (quick): 987,654,321 = 1,000,000,000 − 12,345,679, so
(1e9 − 12,345,679)^2 = 1e18 − 2·12,345,679·1e9 + 12,345,679^2
= 1,000,000,000,000,000,000 − 24,691,358,000,000,000 + 152,415,789,971,041
= 975,461,057,789,971,041.
