Use x² = (x−a)(x+a) + a², choosing a so that x−a and x+a land on numbers you can multiply easily — usually the nearest multiple of ten. This is the same Hint shown in-game.
47² = (47+3)(47−3) + 3²= 50 × 44 + 9= 2209
83² = (83−3)(83+3) + 3²= 80 × 86 + 9= 6889
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Practice 2d²