SOLUTION: What is the value of b if 2x^2 + bx + 8 has a discriminant of -15 ? i said 7 but my teacher marked it wrong !!! I think I should have said -7 or +7 ,can someone please expain this

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Question 118533: What is the value of b if 2x^2 + bx + 8 has a discriminant of -15 ?
i said 7 but my teacher marked it wrong !!! I think I should have said -7 or +7 ,can someone please expain this to me ?
Answer by tutor_paul(519) About Me  (Show Source):
You can put this solution on YOUR website!
The discriminant of a quadratic is the term:
b%5E2-4ac
In your case, a=2, and c=8. Just plug these in and solve for b:
b%5E2-4%2A2%2A8=-15
b%5E2-64=-15
b%5E2=49
highlight%28b=7%29
highlight%28b=-7%29
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Good Luck,
tutor_paul@yahoo.com