Friday, November 13, 2009

`-1, 8, 23, 44, 71, 104....` Decide whether the sequence can be represented perfectly by a linear or a quadratic model. If so, then find the...

The given sequence is:


`-1, 8 , 23, 44, 71, 104`


To determine if it is a linear sequence, take the difference between consecutive terms.


`-1, 8 , 23, 44, 71, 104`


    `vvv` `vvv` `vvv`  `vvv`  `vvv`


    `9` `15``21` `27` `33`


Notice that the result are not the same. So the sequence is not linear.  


Then, take the second difference of the consecutive terms to determine if it is quadratic.


`9, 15, 21, 27, 33`


 `vvv`  `vvv`   `vvv`  `vvv`


 `6`   `6`   `6`   `6`


Since the second difference of the consecutive terms are the same, the sequence is quadratic.


To determine the model of the quadratic expression apply the formula:


`T_n = an^2 + bn + c`


where Tn is the nth term of the sequence.


To solve for the values of a, b and c,  consider the first few three terms of the sequence.


Plug-in T1=-1 and n=1.


`-1=a(1)^2+b(1)+c`


`-1=a+b+c `              (Let this be EQ1.)


Also, plug-in T2=8 and n=2.


`8=a(2)^2 + b(2) + c`


`8=4a+2b+c`           (Let this be EQ2.)


And, plug-in T3=23 and n=3.


`23=a(3)^2+b(3)+c`


`23=9a+3b+c `        (Let this be EQ3.)


Then, apply elimination method of system of equations. Let's eliminate variable c. 


To eliminate c, subtract EQ2 from EQ1.


EQ1:        `-1=a+b+c`


EQ2:      `-(8=4a+2b+c)`


`-----------------`


               `-9 = -3a -b`


And simplify the resulting equation.


`-9=-3a-b`


`9=3a+b `        (Let this be EQ4.)


Eliminate c again. So subtract EQ3 from EQ1.


EQ1:      `-1=a+b+c`


EQ2:  `-(23=9a+3b+c)`


`-----------------`


           `-24=-8a-2b`


And, simplify the resulting equation.


`-24=-8a-2b`


`12=4a+b`        (Let this be EQ5.)


Then, eliminate another variable. Let it be c.


To eliminate c, subtract EQ4 from EQ5.


EQ5:    `12=4a+b`


EQ4:   `-(9=3a+b)`


`--------------`


              `3=a`


Now that the value of a is known, plug-in it to either EQ4 or EQ5. Let's use EQ4.


`9=3a+b`


`9=3(3)+b`


`0=b`


Then, plug-in the value of a and b to either EQ1, EQ2 or EQ3. Let's use EQ1.


`-1=a+b+c`


`-1=3+0+c`


`-4=c`


Now that values of a, b and c, plug-in them to the formula of nth term of quadratic expression.


`T_n=an^2+bn+c`


`T_n=3n^2+(0)n+(-4)`


`T_n=3n^2-4`


Therefore, the given -1, 8 , 23, 44, 71, 104 is a quadratic sequence. The model for its nth term is `T_n=3n^2-4` .


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