Question

p(-4,2), y=-5f(4x)-8 find the corresponding point on the graph to the function.

Answer 1

So I thought x would be 16, and y -18? but it was wrong anyone walk be through this?

Answer 2

@wio

Answer 3

We know f(-4) = 2. This is ALL we know. We know NOTHING of this function except its value at -4.

4x is the argument in the new expression.

If we pick x = -1, we have y = 5f(4(-1)) - 8 = 5f(-4) - 8 = 5(2) - 8 = 10 - 8 = 2

...and we have (x = -1 , y = 2)

Note: We do NOT have f(-1) = 2 -- Not even a little.

Answer 4

Still lost.

Answer 5

Lost by what?

Can you substitute into function notation? I did that. So can you.
Can you replace an expression by its value? I did that. So can you.
Can you manipulate a small collection of numbers and operations? I did that. So can you.

So, what is it that causes you to be lost?

Answer 6

Ok so I get f(-4)=2

I am lost on why we picked x=-1

I though we take the x (so -4) so substitute it in (4(-4)) and that give you the first point?

-16

and for the second point

-5 ( 2 ) =-10 now subtract the -8 giving you -18

so the points would be

(-16,-18)

Answer 7

You do not understand the importance of my first little paragraph, repeated here for convenience:

We know f(-4) = 2. This is ALL we know. We know NOTHING of this function except its value at -4.

We do NOT know f(-16). The ONLY thing we know is f(-4). We do NOT know f(4(-4)). We just don't. The ONLY thing we know is f(-4).

The ONLY way to get f(4) out of f(4x) is to pick x = -1.