Monday, July 9, 2012

What are the coefficients of the function tx^2+(t-1)x-t+2 ?

Since it is not specified if the roots of the equation are
real, distinct, equal or imaginary, we'll choose to solve the problem, considering that
f(x) has 2 distinct real roots.


The equation has 2 distinc
real roots if and only if the discriminant delta is strictly
positive.


delta = b^2 -
4ac


We'll identify a, b and
c.


a = t


b = t -
1


c = 2 - t


delta = (t-1)^2 -
4*t*(2 - t)


We'll impose the constraint: delta >
0


(t-1)^2 - 4*t*(2 - t) >
0


We'll expand the square and remove the
brackets:


t^2 - 2t + 1 - 8t + 4t^2 >
0


We'll combine like
terms:


5t^2 - 10t +
1>0


Now, we'll determine the roots of the expression
5t^2 - 10t + 1.


5t^2 - 10t + 1 =
0


We'll apply the quadratic
formula:


t1 =
[10+sqrt(100-20)]/10


t1 =
(10+4sqrt5)/10


t1 =
(5+2sqrt5)/5


t2 =
(5-2sqrt5)/5


The expression is positive outside the roots,
namely when t belongs to the intervals:(-infinite , (5-2sqrt5)/5) U ((5+2sqrt5)/5 ,
+infinite).


The coefficients of the equation could be
calculated choosing values for t from intervals :(-infinite , (5-2sqrt5)/5) U
((5+2sqrt5)/5 , +infinite).


For  t = -1, the
coefficients of the equation are: a = -1; b = -1 - 1 = -2; c = 2 + 1 =
3.

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