Il faut utiliser l'identité remarquable (a+b)(a-b) = a²-b²
31 * 29 = (30+1) * (30-1) = 30²-1² = 900 - 1 = 899
de même:
49*51= (50-1) * (50+1) = 50²-1² = ...
103 * 97 = (100+3) * (100-3) = 100² - 3² = ...
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Il faut utiliser l'identité remarquable (a+b)(a-b) = a²-b²
31 * 29 = (30+1) * (30-1) = 30²-1² = 900 - 1 = 899
de même:
49*51= (50-1) * (50+1) = 50²-1² = ...
103 * 97 = (100+3) * (100-3) = 100² - 3² = ...