Let a , b , c be three real numbers such that a + b + c = 6 and a b + b c + c a = 9 .
If exactly one root of the equation x 2 − ( m + 2 ) x + 5 m = 0 lies between minimum and maximum value of c , find the number of integral values of m .
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@Brian Charlesworth I agree with you on the first part. Just put a = b to get required values of c . But I believe there is an easier way to do the second part.
Let f ( x ) = x 2 − ( m + 2 ) x + 5 m
We can convince ourselves that if exactly one root of f ( x ) lies in ( α , β ) , then the following condition must be true:
f ( α ) × f ( β ) < 0
In this case, this gives seven values for m . But there is a small catch in this specific situation. It so happens that one of the roots of f ( α ) × f ( β ) = 0 satisfies the given condition. Hence giving us 8 values.
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Yes, for the first stage one can see what the extrema for c (likely) are, but I wanted to be more formal about it. For the second stage, your method would be more succinct; my approach was a bit clumsy, but it was the only way I felt comfortable doing it. I also mistakenly started looking for m that lead to integer values of x , but then once I realized that wasn't what was being asked for I just carried on with the path I was already on to end up, eventually, with the above analysis.
Great solution. I found the conditions on c a different way though.
I considered the curve x 3 − 6 x 2 + 9 x + d (we consider this curve because of Vieta's formulas )and found the x coodinate of the local min and max. Choosing a suitable value for d you can find the min and max value of c .
f ( x ) = x 3 − 6 x 2 + 9 x + d ⟹ f ′ ( x ) = 3 x 2 − 1 2 x + 9 = 3 ( x − 1 ) ( x − 3 )
By considering the graph you can see the min and max for c by varying d
The values of d which give the min and max c are d = 0 and d = − 4 respectively.
0 ≤ c ≤ 4
If anyone wants more detail please just ask. But it's pretty obvious if you sketch a graph.
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I would say its not quite as elegant as yours though.
Cool; I haven't seen it done that way before. Whenever I see equations like the ones in this problem I go straight for Cauchy's, but it's good to know there are other methods one can use. :)
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First note that ( a + b + c ) 2 = a 2 + b 2 + c 2 + 2 ∗ ( a b + b c + c a ) , and thus
a 2 + b 2 + c 2 = 6 2 − 2 ∗ 9 = 1 8 .
Now by Cauchy's Inequality we have that
2 ∗ ( a 2 + b 2 ) ≥ ( a + b ) 2 ⟹ 2 ∗ ( 1 8 − c 2 ) ≥ ( 6 − c ) 2
⟹ 3 6 − 2 c 2 ≥ 3 6 − 1 2 c + c 2 ⟹ 0 ≥ 3 c 2 − 1 2 c
⟹ 0 ≥ 3 c ( c − 4 ) ⟹ 0 ≤ c ≤ 4 .
With c = 0 we have a = b = 3 and with c = 4 we have a = b = 1 . So we have established that the minimum and maximum values of c are 0 and 4 , respectively.
Next, by the quadratic formula the roots of x 2 − ( m + 2 ) x + 5 m are
x = 2 ( m + 2 ) ± ( m + 2 ) 2 − 2 0 m = 2 ( m + 2 ) ± ( m − 8 ) 2 − 6 0 .
Now the first thing we can observe from this result is that, for x to be real for integer m , we require that ( m − 8 ) 2 − 6 0 ≥ 0 , i.e., that either m ≤ 0 or m ≥ 1 6 . Next, applying the " c condition", we require that exactly one of these roots lies on the interval ( 0 , 4 ) . Now for m ≥ 1 6 we know that both roots are positive and one is ≥ 9 so only the choice of the − sign could possibly yield a root in the desired interval. (Note that we know both roots are positive since clearly one is and the product of the roots, namely 5 m , is positive.) So with a bit of algebraic manipulation, we find that we would require that
0 < ( 1 0 + ( ( m − 8 ) − ( m − 8 ) 2 − 6 0 ) < 8 ⟹ 0 < ( 1 0 + A ) < 8 ,
where A = ( m − 8 ) − ( m − 8 ) 2 − 6 0 . But clearly A > 0 for m ≥ 1 6 , and thus the inequality cannot be satisfied, i.e., there are no cases for m ≥ 1 6 where there is exactly one root in the interval ( 0 , 4 ) .
Now we need to look at the m ≤ 0 option. For m = 0 the given quadratic becomes x 2 − 2 x = x ( x − 2 ) = 0 , which, with roots 0 , 2 , does satisfy the condition that exactly one root lies in the interval ( 0 , 4 ) . For m < 0 we know that, since the product of the roots, namely 5 m , is negative, one root will be positive and one negative. So the only chance that one of the roots will lie in the interval ( 0 , 4 ) will be if we choose the + sign in our equation for x . Then with some algebraic manipulation, we find that we would require that
( m + 2 ) + ( m − 8 ) 2 − 6 0 < 8
⟹ ( m − 8 ) 2 − 6 0 < − ( m − 8 ) − 2
⟹ ( m − 8 ) 2 − 6 0 < ( m − 8 ) 2 + 4 ( m − 8 ) + 4
⟹ − 6 4 < 4 ( m − 8 ) ⟹ − 1 6 < ( m − 8 ) ⟹ − 8 < m .
Thus the integral values of m that satisfy the given requirements are those between − 7 and 0 inclusive, giving us a total of 8 integral values.