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А^2 - 7А + 3 = 2
А^2 - 7А = - 1
2 * ( А^2 - 7А ) = 2 * ( - 1 )
2А^2 - 14А = - 2
2А^2 - 14А + 10 = - 2 + 10 = 8
Ответ 8
X^2 - 2x - 15 = (x^2 - 2x + 1) - 16 = (x - 1)^2 - 4^2 = (x - 1 - 4)(x - 1+ 4) = (x - 5)(x + 3)
или через дискриминант:
x^2 - 2x - 15 = 0, D = 4 + 4*15 = 64
x1 = 5, x2 = -3