
The sign of the quadratic polynomial is always positive, if?
A) a is positive and
B) a is positive and
C) a is any real number and
D) a is any real number and
Answer
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Hint: A quadratic polynomial is a parabola on a graph whose concavity depends on the sign of the coefficient of . In order to find the condition for the sign of the quadratic polynomial is always positive, we will apply the following facts:
1) Quadratic polynomial is always positive when it doesn’t intersect the x-axis at any point, so have no real zeroes.
2) In order to get the quadratic polynomial always positive, we need an upward parabola.
Using the above facts, we will find the corresponding results and required conditions.
Complete step by step solution: A quadratic polynomial is a polynomial in a variable (like x) with degree 2. When represented on the graph, a quadratic polynomial is a parabola.
We know that a parabola of the form is a vertical parabola.
Now, it can be an upward parabola or downward parabola.
A downward parabola is the one when y tends to negative infinity for x tending to both positive and negative infinity.
An upward parabola is the one when y tends to positive infinity for x tending to both positive and negative infinity.
Thus, in order to get the quadratic polynomial always positive, it must be an upward parabola.
Now we know that, for a quadratic polynomial ,
represents an upward parabola whereas represents a downward parabola.
Thus, in order to have the quadratic polynomial always positive or an upward parabola, we must have,
…(i)
Now, we know that
A quadratic polynomial is a polynomial in a variable (like x) with degree 2, thus, it will have 2 zeroes, real or imaginary.
Now, in order to have the quadratic polynomial always positive, it must always remain above the x-axis,
Or there must not be an x for which the quadratic polynomial is zero or negative.
Thus, the quadratic polynomial has imaginary zeroes
Now, for the quadratic equation
The condition for having no real roots is its discriminant must be less than zero.
Discriminant of a quadratic equation is defined as:
Now, to get discriminant of a quadratic equation less than zero,
Thus, …(ii)
From (i) and (ii), we get,
and
Hence, most suitable option is option a and
Note: Another approach to the question can be solving by the graph of the polynomial. For the sign of the quadratic polynomial to be always positive, we need an upward parabola, which we will get when . So, it is our first condition.
Now, we need the parabola to never cut axis, so its vertex should be above the x-axis.
Now, vertex of a parabola is
So, y-coordinate of vertex must be greater than zero.
Now,
Hence, the required conditions are, and
1) Quadratic polynomial
2) In order to get the quadratic polynomial
Using the above facts, we will find the corresponding results and required conditions.
Complete step by step solution: A quadratic polynomial is a polynomial in a variable (like x) with degree 2. When represented on the graph, a quadratic polynomial is a parabola.
We know that a parabola of the form
Now, it can be an upward parabola or downward parabola.
A downward parabola is the one when y tends to negative infinity for x tending to both positive and negative infinity.
An upward parabola is the one when y tends to positive infinity for x tending to both positive and negative infinity.
Thus, in order to get the quadratic polynomial
Now we know that, for a quadratic polynomial
Thus, in order to have the quadratic polynomial
Now, we know that
A quadratic polynomial is a polynomial in a variable (like x) with degree 2, thus, it will have 2 zeroes, real or imaginary.
Now, in order to have the quadratic polynomial
Or there must not be an x for which the quadratic polynomial
Thus, the quadratic polynomial
Now, for the quadratic equation
The condition for having no real roots is its discriminant must be less than zero.
Discriminant of a quadratic equation is defined as:
Now, to get discriminant of a quadratic equation less than zero,
Thus,
From (i) and (ii), we get,
Hence, most suitable option is option a
Note: Another approach to the question can be solving by the graph of the polynomial. For the sign of the quadratic polynomial
Now, we need the parabola to never cut axis, so its vertex should be above the x-axis.
Now, vertex of a parabola
So, y-coordinate of vertex must be greater than zero.
Now,
Hence, the required conditions are,
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