I need help with this:
http://prntscr.com/bsupx2
thanks!

Question

I need help with this:
http://prntscr.com/bsupx2
thanks!

oswaldmurphy · Answer

@.Sam.

oswaldmurphy · Answer

@mathstudent55 @sammixboo @zepdrix

zootedyoungin · Answer

Graph C

zootedyoungin · Answer

Probably shouldnt have given you a direct answer

oswaldmurphy · Answer

why is it graph C? its fine as long as you explain.

mathstudent55 · Answer

Let x = 0.
What do you get for y?

oswaldmurphy · Answer

1 or 5?

mathstudent55 · Answer

$y = f(x) = 5(2)^{-x} $

$y = f(0) = 5(2)^{-0} = 5(1) = 5 $

oswaldmurphy · Answer

Oh ok.

mathstudent55 · Answer

This tells you point (0, 5) must be on the graph.
Which solutions can you already eliminate?

oswaldmurphy · Answer

C and D

mathstudent55 · Answer

Correct.
We need to choose from A and B, both of which pass by point (0, 5).

mathstudent55 · Answer

Let x = 1.
What do youi get for y?

oswaldmurphy · Answer

a negative and a is going down, so A?

mathstudent55 · Answer

$y = f(x) = 5(2)^{-x} $

$y = f(1) = 5(2)^{-1} = 5(0.5) = 2.5$

mathstudent55 · Answer

Now we have point (1, 2.5) which confirms the answer is A.
Incidentally, as soon as we got point (0, 5), we would be able to tell the answer with certainty.
We needed a graph that showed decay and that went through (0, 5).
Graph B goes through (0, 5) but shows growth (y increases as x increases), not decay, so the only possible correct answer was A.