Past Papers’ Solutions  Assessment & Qualification Alliance (AQA)  AS & A level  Mathematics 6360  Pure Core 1 (6360MPC1)  Year 2012  June  Q#3
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Question
The polynomial is given by .
a.
i. Use the Factor Theorem to show that is a factor of .
ii. Express as the product of three linear factors.
b. Verify that .
c. Sketch the curve with equation , indicating the values where the curve crosses the xaxis.
Solution
a.
i.
Factor theorem states that if is a factor of then;
For the given case is factor of .
We can write the divisor in standard form as;
Here and . Hence;
Hence, is factor of .
ii.
If is a polynomial of degree then will have exactly factors, some of which may repeat.
We are given a polynomial of degree ,
Therefore, it will have 03 factors some of which may repeat.
From (a:i) we already have 01 factor of .
We may divide the given polynomial with any of the already known linear factor(s) to get a quadratic factor and then factorize the obtained quadratic factor to find the 2^{nd} and 3^{rd} linear factors.
For the given case;
Already known factor from (a:i) is .
We may divide the given polynomial by factor .
Therefore, we get the quadratic factor for given polynomial. Now we factorize this quadratic factor.
Hence, can be written as product of three linear factors as follows;
b.
We are given that;
We are required to verify that;
Hence .
c.
We are required to sketch a cubical polynomial given as;
ü Find the sign of the coefficient of . This gives the shape of the graph at the extremities. If the coefficient of is such that cubic graph is increasing from left to right at the extremities and when the cubic graph is inverted.
The coefficient of in the given polynomial is positive. Therefore it will cause normal cubic graph increasing from left to right at the extremities.
ü Find the point where the graph crosses yaxis by finding the value of when .
Therefore for ;
Hence, the cubic graph intercepts yaxis at .
ü Find the point(s) where the graph crosses the xaxis by finding the value of when . If there is repeated root the graph will touch the xaxis.
From (a:ii) we know that given polynomial can be written as;
It is evident from factors that the function will cross/intersect horizontal axis when;









It is evident cubic graph will intersect xaxis at .
ü Calculate the values of for some value of . The is particularly useful in determining the quadrant in which the graph might turn close to the yaxis.
ü Complete the sketch of the graph by joining the sections.
Sketch should show the main features of the graph and also, where possible, values where the graph intersects coordinate axes.
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