SOLUTION: The function f is defined by f(x)=x^3 - 4x. The function of g is defined by g(x)=f(x+h)+k where h and k are constants. What is the value of hk? Points for y=f(x)are (-1,3),

Algebra ->  College  -> Linear Algebra -> SOLUTION: The function f is defined by f(x)=x^3 - 4x. The function of g is defined by g(x)=f(x+h)+k where h and k are constants. What is the value of hk? Points for y=f(x)are (-1,3),      Log On


   



Question 470687: The function f is defined by f(x)=x^3 - 4x. The function of g is defined by g(x)=f(x+h)+k where h and k are constants. What is the value of hk?
Points for y=f(x)are (-1,3), (1,-3)
Points for y=g(x) are (2,1), (4,-5)
The answer is 6, but I do not understand how they came up with this answer. Please help!

Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

First y+=+f%28x%29+and f%28x%29+=+x%5E3+-+4x, y+=+g%28x%29+and g%28x%29+=+f%28x+%2B+h%29+%2B+k or
y+=+f%28x%29+=+x%5E3+-4x
y+=+g%28x%29+=+f%28x+%2B+h%29+%2B+k
Looks to me like we have two equations for y which means we can solve them as a system. Notice that g%28x%29+contains f%28x%29 but with a different input variable, namely %28x+%2B+h%29.
To find f%28x+%2B+h%29 we replace+x with %28x+%2B+h%29 wherever we find an x in the f equation.
f%28x%29+=+x%5E3+-+4x will become
f%28x+%2B+h%29+=+%28x+%2B+h%29%5E3+-+4%28x+%2B+h%29
So now we replace f%28x+%2B+h%29 in g%28x%29 with our new result.
g%28x%29+=+f%28x+%2B+h%29+%2Bk
g%28x%29+=+%28+%28x+%2B+h%29%5E3+-+4%28x+%2B+h%29+%29+%2B+k
Also, we are given a pair of points that work with g%28x%29 so lets plot them in and get a pair of equations. The first number in each point is x+and the second is y.
Points in g(x) : (2, 1) and (4, -5)
So for the first :
1+=+%282+%2B+h%29%5E3+-+4%282+%2B+h%29+%2B+k
1+=+%288+%2B+12h+%2B+6h%5E2+%2B+h%5E3%29+-+%288+%2B+4h%29+%2B+k
1+=+8+%2B+12h+%2B+6h%5E2+%2B+h%5E3+-+8+-+4h+%2B+k
1+=+h%5E3+%2B+6h%5E2+%2B+8h+%2B+k
1+-+%28h%5E3+%2B+6h%5E2+%2B+8h%29+=+k
-+h%5E3+-+6h%5E2+-+8h+%2B+1+=+k
and for the second :
-5+=+%284+%2B+h%29%5E3+-+4%284+%2B+h%29+%2B+k
-5+=+%2864+%2B+48h+%2B+12h%5E2+%2B+h%5E3%29+-+%2816+%2B+4h%29+%2B+k
-5+=+64+%2B+48h+%2B+12h%5E2+%2B+h%5E3+-+16+-+4h+%2B+k
-5+=+h%5E3+%2B+12h%5E2+%2B+44h+%2B+48+%2B+k
-5+-+%28h%5E3+%2B+12h%5E2+%2B+44h+%2B+48%29+=+k
-+h%5E3+-+12h%5E2+-+44h+-+53+=+k
Notice that both long equations are equal to k. That means they are also equal to each other.
-+h%5E3+-+6h%5E2+-+8h+%2B+1+=+-+h%5E3+-+12h%5E2+-+44h+-+53
-+h%5E3+-+6h%5E2+-+8h+%2B+1+-+%28-+h%5E3+-+12h%5E2+-+44h+-+53%29+=+0
-+h%5E3+-+6h%5E2+-+8h+%2B+1+%2B+h%5E3+%2B+12h%5E2+%2B+44h+%2B+53+=+0
6h%5E2+%2B+36h+%2B+54+=+0
6%28h%5E2+%2B+6h+%2B+9%29+=+0
%28h+%2B+3%29%28h+%2B+3%29+=+0+%2F+6
h+%2B+3+=+0
h+=+-3
So+h is -3, now we need k.
g%28x%29+=+%28+%28x+%2B+h%29%5E3+-+4%28x+%2B+h%29+%29+%2B+k
y+=+%28+%28x+%2B+h%29%5E3+-+4%28x+%2B+h%29+%29+%2B+k
1+=%28+%282+-+3%29%5E3+-+4%282+-+3%29%29+%2B+k
1+=+-1%5E3+-+4%28-1%29+%2B+k
1+=+-1+%2B+4+%2B+k
1+=+3+%2B+k
-2+=+k
So k+is -2. We already know that h is -3. so :
hk+=+-3%2A-2=6