-x^2 +6x +7 = 0
x^2 - 6x - 7 = 0
D = 36 -4(-7) = 36 + 28 = 64; √D = 8
x1 = (6 + 8)/2 = 7
x2 = (6 - 8)/2 = - 1
Ответ: второй сверху.
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-х^2 + 10x - 15 = 0
x^2 - 10x + 15 = 0
D = 100 - 4(15) = 100 - 60 = 40; √D = 2√10
x1 = (10 + 2√10)/ 2 = 2(5 +√10)/2 = 5 +√10
x2 = (10 - 2√10)/2 = 2(5 - √10)/2 = 5 - √10
Ответ: второй снизу