John Oon's answer to lana <3's Secondary 3 A Maths Singapore question.
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When a less than zero the quadratic function concave down.
Date Posted:
2 years ago
(ii) asked for an example of a & c which satisfy (I).
thank you ! helped a lot !
sorry to disturb, but could you just explain why a<0 and ac>81/16 ? I don’t quite understand how it came to that conclusion. Thank you in advance !
a < 0 because that makes the quadratic function concave down. Only when the quadratic function concave down it is possible to have the curve always below the x- axis.
ac > 81/16 is derived from Discriminate < 0
When discriminate < 0
We can use the formula : b^2-4ac
b = 9
a = 4a
c = c
So 9^2 - 4(4a)(c) < 0
81 - 16ac < 0
81 < 16ac
ac > 81/16
ac > 81/16 is derived from Discriminate < 0
When discriminate < 0
We can use the formula : b^2-4ac
b = 9
a = 4a
c = c
So 9^2 - 4(4a)(c) < 0
81 - 16ac < 0
81 < 16ac
ac > 81/16
alright ! understood. thank you very much ! you’ve been very helpful <3
You are welcome
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