## Function Correspondence: (1, 8) is on the graph of y = f(x)

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lordofthefishes
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### Function Correspondence: (1, 8) is on the graph of y = f(x)

"The point (1,8) is on the graph y=f(x). Find the corresponding coordinates of this point on each of the following graphs:

... c) y= -2f(-x)-7."

The textbook answer is (-1,9). I don't understand how they got this answer. The only way that that would make sense is if the corresponding y coordinate was doubled and positive, then subtracted by 7. But why would it be positive? The equation for corresponding coordinates is (x/k+d,ya+c) (where the parent function is y= af(k(x-d))+c). Accordingly, the y coordinate should be negative. Specifically, I think it should be -23.

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### Re: Function Correspondence: (1, 8) is on the graph of y = f

"The point (1,8) is on the graph y=f(x). Find the corresponding coordinates of this point on each of the following graphs:

... c) y= -2f(-x)-7."

The textbook answer is (-1,9). I don't understand how they got this answer.
They gave you (x,y) = (1,8). The new one is f(-x) so you have to do -1 so -x = -(-1) = +1 = 1.

Check w/ your teacher. I think there's maybe a mistake and they should have given you (1,-8) because -2(-8)-7 = 16-7=9.

lordofthefishes
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### Re: Function Correspondence: (1, 8) is on the graph of y = f

Thank you, but I'm still a little confused. Why "do I have to do -1 so -x = -(-1) = +1 = 1"? Shouldn't the x correspondence of y = f(x) be (1)/-1 = -1 according to the correspondence equation?

buddy
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### Re: Function Correspondence: (1, 8) is on the graph of y = f

Thank you, but I'm still a little confused. Why "do I have to do -1 so -x = -(-1) = +1 = 1"? Shouldn't the x correspondence of y = f(x) be (1)/-1 = -1 according to the correspondence equation?
Whichever. It works out the same either way. If you look at "what does the new input need to be, so you get the one they gave you?" then you have to do "-1" becuase they gave you f(x) for x = 1 and then they asked you about f(-x). The only x you had was x = 1 so you had to do something to make "-x" in f(-x) equal the x= 1 they gave you. Or you can plug into your formula. Either way gets you to putting -1 in for your new correspondence point.