Answer: b = 14.
Step-by-step 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.
