Expert: Scott A Wilson - 9/22/2010

Question
daughter is in 6th grade.having trouble in math, algebra.
doing function tables X = 2,5,8,12.  y = 1,7,13,?
what is the rule of this function?
what is missing number? and
if X=20 what is value of Y?

I understand the concept of function table but can't figure this out

Answer
The first three x values are 2, 5, 8 and the first three y values area 1, 7, 13.  Notice that for the x values, there is an increase of 3 each time, for 2+3 = 5 and 5+3=8.  For the y values, there is an increase of 6 each time, for 1+6=7 and 7+6=13.

This means that each time x increases by 3, y increases by 6.
In general, y increases at twice the rate of x.

The differene in the last two x values is 12-8=4, so the next y value would be found by taking the x difference of 4 times 2 and adding it to the last y value.  That is, 2*4 + 13 = 21.

In general, take 2x for the first three x values given of 2, 5, 8
and get 4, 10, 16.  Note that if 3 is subtracted, we get 4-3=1,
10-3=7, and 16-3=13, so we get 1, 7, and 13, which is the y values,

So the rule for this function is y = 2x - 3.

This means if we take x=20, we get 2(20) - 3 = 40 - 3 = 37,
and that is the value of Y at this point.