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The sign of the quadratic polynomial ax2+bx+c is always positive, if?
A) a is positive and b24ac0
B) a is positive and b24ac0
C) a is any real number and b24ac0
D) a is any real number and b24ac0

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 x2. In order to find the condition for the sign of the quadratic polynomial ax2+bx+c is always positive, we will apply the following facts:
1) Quadratic polynomial ax2+bx+c 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 ax2+bx+calways 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 y=ax2+bx+c 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 ax2+bx+c always positive, it must be an upward parabola.
Now we know that, for a quadratic polynomialax2+bx+c,
a>0 represents an upward parabola whereas a<0 represents a downward parabola.
Thus, in order to have the quadratic polynomial ax2+bx+c always positive or an upward parabola, we must have,
a>0 …(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 ax2+bx+c always positive, it must always remain above the x-axis,
Or there must not be an x for which the quadratic polynomial ax2+bx+cis zero or negative.
Thus, the quadratic polynomial ax2+bx+c has imaginary zeroes
Now, for the quadratic equation ax2+bx+c=0
The condition for having no real roots is its discriminant must be less than zero.
Discriminant of a quadratic equation is defined as: D=b24ac
Now, to get discriminant of a quadratic equation less than zero, b24ac<0
Thus, b24ac<0 …(ii)
From (i) and (ii), we get,
a>0 and b24ac<0
Hence, most suitable option is option a a>0 and b24ac0

Note: Another approach to the question can be solving by the graph of the polynomial. For the sign of the quadratic polynomial ax2+bx+c to be always positive, we need an upward parabola, which we will get when a>0. 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 y=ax2+bx+c is (b2a,4acb24a)
So, y-coordinate of vertex must be greater than zero.
4acb24a>0
Now, a>0
4acb2>0
b24ac<0
Hence, the required conditions are, a>0 and b24ac<0
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