- #1
VinnyCee
- 489
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Find a polynomial of degree < 2 [a polynomial of the form [itex]f\left(t\right)\,=\,a\,+\,bt\,+\,ct^2[/itex]] whose graph goes through the points (1, p), (2, q), (3, r), where p, q, r are arbitrary constants. Does such a polynomial exist for all values of p, q, r?
Here is what I have so far:
(1, p) ==> [itex]p\,=\,a\,+\,b\,+\,c[/itex]
(2, q) ==> [itex]q\,=\,a\,+\,2b\,+\,4c[/itex]
(3, r) ==> [itex]r\,=\,a\,+\,3b\,+\,9c[/itex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
1 & 2 & 4 & q \\
1 & 3 & 9 & r
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_2\,=\,R_2\,-\,R_1[/tex]
[tex]R_3\,=\,R_3\,-\,R_1[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 3 & q - p \\
0 & 2 & 8 & r - p
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_3\,=\,R_3\,-\,2R_2[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 3 & q - p \\
0 & 0 & 2 & r - q
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_3\,=\frac{1}{2}\,R_3[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 3 & q - p \\
0 & 0 & 1 & \frac{1}{2}\left(r - q\right)
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_2\,=\,R_2\,-\,3R_3[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 0 & \frac{5}{2}\,q\,-\,p\,-\,\frac{3}{2}\,r \\
0 & 0 & 1 & \frac{1}{2}\left(r - q\right)
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_1\,=\,R_1\,-\,R_2\,-\,R_3[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 0 & 0 & 2p\,-\,2q\,+\,r \\
0 & 1 & 0 & \frac{1}{2}\left(5q\,-\,2p\,-\,3r\right) \\
0 & 0 & 1 & \frac{1}{2}\left(r - q\right)
\end{array} \right]
\end{displaymath}[/tex]
[tex]a\,=\,2p\,-\,2q\,+\,r[/tex]
[tex]b\,=\,\frac{1}{2}\,\left(5q\,-\,2p\,-\,3r\right)[/tex]
[tex]c\,=\,\frac{1}{2}\,\left(r\,-\,q\right)[/tex]
[tex]f\left(t\right)\,=\,a\,+\,bt\,+\,ct^2[/tex]
[tex]f\left(t\right)\,=\,\left(2p\,-\,2q\,+\,r\right)\,+\,\left[\frac{1}{2}\,\left(5q\,-\,2p\,-\,3r\right)\right]\,t\,+\,\left[\frac{1}{2}\,\left(r\,-\,q\right)\right]\,t^2[/tex]
[tex]f\left(t\right)\,=\frac{1}{2}\,r\,t^2\,-\,\frac{1}{2}\,q\,t^2\,+\,\frac{5}{2}\,q\,t\,-\,\frac{3}{2}\,r\,t\,-\,p\,t\,+\,2\,p\,+\,r\,-\,2\,q[/tex]
But when I try a set of arbitrary numbers [(1, 8), (2, 12), (3, 3)], I get an equation that does not match the points:
[tex]f\left(t\right)\,=\,-\frac{9}{2}\,t^2\,+\,\frac{35}{2}\,t\,-\,5[/tex]
[tex]f\left(1\right)\,=\,-\frac{9}{2}\,(1)^2\,+\,\frac{35}{2}\,(1)\,-\,5\,=\,8[/tex]
[tex]f\left(2\right)\,=\,-\frac{9}{2}\,(2)^2\,+\,\frac{35}{2}\,(2)\,-\,5\,=\,12[/tex]
[tex]f\left(3\right)\,=\,-\frac{9}{2}\,(3)^2\,+\,\frac{35}{2}\,(3)\,-\,5\,=\,7[/tex]
However, [itex]f\left(3\right)[/itex] should be equal to r, which is 3, not 7.
Please help:)
Here is what I have so far:
(1, p) ==> [itex]p\,=\,a\,+\,b\,+\,c[/itex]
(2, q) ==> [itex]q\,=\,a\,+\,2b\,+\,4c[/itex]
(3, r) ==> [itex]r\,=\,a\,+\,3b\,+\,9c[/itex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
1 & 2 & 4 & q \\
1 & 3 & 9 & r
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_2\,=\,R_2\,-\,R_1[/tex]
[tex]R_3\,=\,R_3\,-\,R_1[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 3 & q - p \\
0 & 2 & 8 & r - p
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_3\,=\,R_3\,-\,2R_2[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 3 & q - p \\
0 & 0 & 2 & r - q
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_3\,=\frac{1}{2}\,R_3[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 3 & q - p \\
0 & 0 & 1 & \frac{1}{2}\left(r - q\right)
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_2\,=\,R_2\,-\,3R_3[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 1 & 1 & p \\
0 & 1 & 0 & \frac{5}{2}\,q\,-\,p\,-\,\frac{3}{2}\,r \\
0 & 0 & 1 & \frac{1}{2}\left(r - q\right)
\end{array} \right]
\end{displaymath}[/tex]
[tex]R_1\,=\,R_1\,-\,R_2\,-\,R_3[/tex]
[tex]\begin{displaymath}
\left[ \begin{array}{ccc|c}
1 & 0 & 0 & 2p\,-\,2q\,+\,r \\
0 & 1 & 0 & \frac{1}{2}\left(5q\,-\,2p\,-\,3r\right) \\
0 & 0 & 1 & \frac{1}{2}\left(r - q\right)
\end{array} \right]
\end{displaymath}[/tex]
[tex]a\,=\,2p\,-\,2q\,+\,r[/tex]
[tex]b\,=\,\frac{1}{2}\,\left(5q\,-\,2p\,-\,3r\right)[/tex]
[tex]c\,=\,\frac{1}{2}\,\left(r\,-\,q\right)[/tex]
[tex]f\left(t\right)\,=\,a\,+\,bt\,+\,ct^2[/tex]
[tex]f\left(t\right)\,=\,\left(2p\,-\,2q\,+\,r\right)\,+\,\left[\frac{1}{2}\,\left(5q\,-\,2p\,-\,3r\right)\right]\,t\,+\,\left[\frac{1}{2}\,\left(r\,-\,q\right)\right]\,t^2[/tex]
[tex]f\left(t\right)\,=\frac{1}{2}\,r\,t^2\,-\,\frac{1}{2}\,q\,t^2\,+\,\frac{5}{2}\,q\,t\,-\,\frac{3}{2}\,r\,t\,-\,p\,t\,+\,2\,p\,+\,r\,-\,2\,q[/tex]
But when I try a set of arbitrary numbers [(1, 8), (2, 12), (3, 3)], I get an equation that does not match the points:
[tex]f\left(t\right)\,=\,-\frac{9}{2}\,t^2\,+\,\frac{35}{2}\,t\,-\,5[/tex]
[tex]f\left(1\right)\,=\,-\frac{9}{2}\,(1)^2\,+\,\frac{35}{2}\,(1)\,-\,5\,=\,8[/tex]
[tex]f\left(2\right)\,=\,-\frac{9}{2}\,(2)^2\,+\,\frac{35}{2}\,(2)\,-\,5\,=\,12[/tex]
[tex]f\left(3\right)\,=\,-\frac{9}{2}\,(3)^2\,+\,\frac{35}{2}\,(3)\,-\,5\,=\,7[/tex]
However, [itex]f\left(3\right)[/itex] should be equal to r, which is 3, not 7.
Please help:)
Last edited: