Question

5. The quadratic expression 21 + 12x - 4x2 takes
(a) the least value 5
(6) the highest value 30
(c) the highest value 21
(d) none of these

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Answer 1

Answer:

d is the correct answer

Answer 2

The correct option is b.

Step-by-step explanation:

The given quadratic expression is

$21+12x-4x^2$

Here, the leading coefficient is -4 which is a negative number. So, the given quadratic expression represents a downward parabola.

The vertex of a downward parabola is the point of maxima.

If a quadratic function is $f(x)=ax^2+bx+c$, then

$Vertex=(-\dfrac{b}{2a},f(-\dfrac{b}{2a}))$

In the given expression a=-4,b=12 and c=21.

$-\dfrac{b}{2a}=-\dfrac{12}{2(-4)}=1.5$

Substitute 1.5 in the given expression to find the highest value.

$21+12(1.5)-4(1.5)^2=30$

The highest value of the given expression is 30.

Therefore, the correct option is b.

#Learn more

The axis of symmetry for a function in the form f(x) = x2 + 4x − 5 is x = −2. What are the coordinates of the vertex of the graph?

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