Question

Write an exponential function for graph that passes through the following points (-3,80);(-1,20)

Answer 1

Answer:

y

=

4

(

1

2

)

x

Explanation:

An exponential function is in the general form

y

=

a

(

b

)

x

We know the points

(

−

1

,

8

)

and

(

1

,

2

)

, so the following are true:

8

=

a

(

b

−

1

)

=

a

b

2

=

a

(

b

1

)

=

a

b

Multiply both sides of the first equation by

b

to find that

8

b

=

a

Plug this into the second equation and solve for

b

:

2

=

(

8

b

)

b

2

=

8

b

2

b

2

=

1

4

b

=

±

1

2

Two equations seem to be possible here. Plug both values of

b

into the either equation to find

a

. I'll use the second equation for simpler algebra.

If

b

=

1

2

:

2

=

a

(

1

2

)

a

=

4

Giving us the equation:

y

=

4

(

1

2

)

x

If

b

=

−

1

2

:

2

=

a

(

−

1

2

)

a

=

−

4

Giving us the equation:

y

=

−

4

(

−

1

2

)

x

However! In an exponential function,

b

>

0

, otherwise many issues arise when trying to graph the function.

The only valid function is

y

=

4

(

1

2

)

x