Question

Find the values of b such that the function has the given minimum value.

f(x) = x2 + bx − 21;
Minimum value:
−70

Answer 1

Answer: b = 14.

Explanation:

We have the function:

f(x) = x^2 + b*x - 21

The minimum of a quadratic function will be at the vertex, and the vertex is at the value of x such that:

f'(x) = 2*x + b = 0

then the vertex is at:

x = -b/2

Then the minimum of the function f(x) will be:

f(-b/2) = (-b/2)^2 + b*(-b/2) - 21

and we know that the minimum value is -70, then:

f(-b/2) = -70 = (b^2)/4 - (b^2)/2 - 21

Now we need to solve this equation for b.

-70 = (b^2)/4 - (b^2)/2 - 21

-70 = -(b^2)/4 - 21

-70 + 21 = -(b^2)/4

49 = (b^2)/4

49*4 = b^2

196 = b^2

√196 = b = 14

The value of b is 14.