Question

Ind an equation for the nth term of the arithmetic sequence.

a14 = -33, a15 = 9

Answer 1

`a_n=42n-621`

Explanation:

This is arithmetic sequence so you should go to linear equations in your head.

Think of the question asking you to find the equation of line going through the points:

(14,-33) and (15,9).

First, let's find the slope.

You need to compute y's change over x's change.

The way I like to do that is line up the points vertically, subtract them, and then put 2nd difference over first.

Like this:

( 15 , 9)

-( 14 , -33)

------------------

1 42

So the slope is 42/1=42.

(The slope is our common difference.)

Now point slope form is:

y-y1=m(x-x1) where m is the slope and (x1,y1) is a point you know on the line.

So we have m=42 and (x1,y1)=(15,9). (You could have chose the other point.)

y-9=42(x-15)

I'm going to put in slope-intercept form. Slope-intercept form is y=mx+b where m is the slope and b is the y-intercept.

y-9=42(x-15)

Solve for y by adding 9 on both sides:

y=42(x-15)+9

Distribute 42 to terms in the ( ) :

y=42x-42(15)+9

y=42x-630+9

y=42x-621

So we can which back to n now:

`a_n=42n-621`