nonameee wrote:
1) Is there a quick way to know whether a quadratic equation has one or two positive solutions without the need to solve it?
The only thing that comes to mind is to calculate (or rather to approximate) a discriminant and to compare it with b as in: ax^2 + bx + c = 0
Unless the equation can be solved quickly by inspection, I think that is the
only way to know if a quadratic equation has two positive solutions.
For example, it is pretty easy to solve the equation x^2 + 4x - 5 by inspection: (x - 1)(x + 5)
Therefore, the solutions are 1 and -5.
However, if the solution cannot be found quickly by inspection, then you have to calculate the discriminant, D: D =
b^2 - 4
acIf D = 0, the equation has two real, equal roots. But you don't know if they are positive without also considering the signs of
a and
b.
If D <> 0, you have no choice but to compute √D and compare it to
b and again taking into consideration the signs of numerator and denominator.
nonameee wrote:
2) Is there a quick way to know whether a quadratic equation has whole solutions?
See my response for #1 above. As far as I know, unless a quick solution can be determined by inspection, you'd have to go through the quadratic formula. (Or draw a quick graph. Or differentiate the equation and see where a maximum occurs and perhaps noting that both zeros are on one side of the y-axis. But I think either of these approaches would be more hassle.)
nonameee wrote:
3) Is there a quick way to know whether the following equation has one positive whole solution:
\(\frac{60}{n}=\frac{60}{n-5}-2\)
Again, there is no
quick way to determine this.
You'd have to give all terms in this equation a common denominator and then gather like terms. You'll end up with a quadratic equation:
n^2 - 5n - 150 (assuming I have done it correctly)
Fortunately, this equation can be solved quickly by inspection: (n + 10)(n - 15)
So, yes, this equation has one positive root: 15