MHB Evaluate 80t - 20s + 60r - 50q + 70p

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The discussion revolves around evaluating the expression 80t - 20s + 60r - 50q + 70p using a system of simultaneous equations. A linear algebra approach is suggested, involving the transformation of the equations into matrix form and deriving relationships between the variables. After calculating the values of p, q, r, s, and t, the initial result is found to be 685. However, an error in the order of the equations is identified, necessitating a swap between the values of q and r, which impacts the final calculation. The conversation highlights the importance of careful reading in problem-solving and the value of alternative methods.
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Given a system of simultaneous equations below:

$pqrs+pqrt+pqst+prst=-264$

$pqrs+pqrt+prst+qrst=-24$

$pqrs+pqrt+pqst+qrst=24$

$pqrs+pqst+prst+qrst=248$

$pqrt+pqst+prst+qrst=-16$

Evaluate $80t-20s+60r-50q+70p$.
 
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Re: Evaluate 80t-20s+60r-50q+70p

[sp]I think that the best way to do this is to use some linear algebra. Divide the equations by $pqrst$ and write them in matrix form: $$\begin{bmatrix}0&1&1&1&1 \\ 1&0&1&1&1 \\ 1&1&0&1&1 \\ 1&1&1&0&1 \\ 1&1&1&1&0 \end{bmatrix} \begin{bmatrix}p^{-1} \\q^{-1} \\r^{-1} \\s^{-1} \\t^{-1} \end{bmatrix} = \frac1{pqrst}\begin{bmatrix}-264 \\ -24 \\ 24 \\ 248 \\ -16 \end{bmatrix}.$$ That $5\times5$ matrix is equal to $5P-I$, where $P$ is the matrix of the projection onto the 1-dimensional subspace spanned by the unit vector $\frac1{\sqrt5}(1,1,1,1,1)$. The inverse of that matrix is $\tfrac54P-I = \dfrac14 \begin{bmatrix}-3&1&1&1&1 \\ 1&-3&1&1&1 \\ 1&1&-3&1&1 \\ 1&1&1&-3&1 \\ 1&1&1&1&-3 \end{bmatrix}.$ It follows that $$\begin{bmatrix}p^{-1} \\q^{-1} \\r^{-1} \\s^{-1} \\t^{-1} \end{bmatrix} = \frac1{4pqrst} \begin{bmatrix}-3&1&1&1&1 \\ 1&-3&1&1&1 \\ 1&1&-3&1&1 \\ 1&1&1&-3&1 \\ 1&1&1&1&-3 \end{bmatrix} \begin{bmatrix}-264 \\ -24 \\ 24 \\ 248 \\ -16 \end{bmatrix} = \frac1{pqrst}\begin{bmatrix}256 \\ 16 \\ -32 \\ -256 \\ 8 \end{bmatrix} = \frac1{pqrst}\begin{bmatrix}2^8 \\ 2^4 \\ -2^5 \\ -2^8 \\ 2^3 \end{bmatrix}.$$ If $\Pi = pqrst$ then $p^{-1} = \dfrac{2^8}{\Pi}$, $q^{-1} = \dfrac{2^4}{\Pi}$, $r^{-1} = -\dfrac{2^5}{\Pi}$, $s^{-1} = -\dfrac{2^8}{\Pi}$, $t^{-1} = \dfrac{2^3}{\Pi}.$ Multiply those five equations together to see that $\dfrac1{\Pi} = \dfrac{2^{28}}{\Pi^5}$, from which $\Pi^4 = 2^{28}$ and so $\Pi = 2^7$.

Thus $p = 1/2$, $q = 8$, $r = -4$, $s = -1/2$, $t = 16.$ If I have done the arithmetic correctly, then $80t-20s+60r-50q+70p = 685.$[/sp]

Edit. The left sides of the original equations each contain four of the five products $pqrs$, $pqrt$, $pqst$, $prst$, $qrst$. When I first read the problem, I saw that the first equation uses the four products that include $p$, and the last equation uses the four products that include $t$. I assumed, without stopping to read everything carefully, that if the first equation was a "$p$" equation and the last equation was a "$t$" equation, then the intermediate equations would be the "$q$", "$r$", "$s$" equations, in that order. But anemone points out to me in a PM that the order of the "$q$" and "$r$" equations is reversed. This means that my values for $q$ and $r$ should be exchanged, and that obviously alters the result of the final calculation.
 
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Re: Evaluate 80t-20s+60r-50q+70p

Thanks for participating, Opalg! Even though you have read the problem wrongly, I am glad to learn that there is another way (which is different than mine) to approach the problem.:o

My solution:
Divide all given equations by $pqrst$ we get:

$\dfrac{pqrs}{pqrst}+\dfrac{pqrt}{pqrst}+\dfrac{pqst}{pqrst}+\dfrac{prst}{pqrst}=-\dfrac{264}{pqrst}\rightarrow\;\dfrac{1}{t}+\dfrac{1}{s}+\dfrac{1}{r}+\dfrac{1}{q}=-\dfrac{264}{pqrst}-(1)$

$\dfrac{pqrs}{pqrst}+\dfrac{pqrt}{pqrst}+\dfrac{prst}{pqrst}+\dfrac{qrst}{pqrst}=-\dfrac{24}{pqrst} \rightarrow\;\dfrac{1}{t}+\dfrac{1}{s}+\dfrac{1}{q}+\dfrac{1}{p}=-\dfrac{24}{pqrst}-(2)$

$\dfrac{pqrs}{pqrst}+\dfrac{pqrt}{pqrst}+\dfrac{pqst}{pqrst}+\dfrac{qrst}{pqrst}=\dfrac{24}{pqrst} \rightarrow\;\dfrac{1}{t}+\dfrac{1}{s}+\dfrac{1}{r}+\dfrac{1}{p}=\dfrac{24}{pqrst}-(3)$

$\dfrac{pqrs}{pqrst}+\dfrac{pqst}{pqrst}+\dfrac{prst}{pqrst}+\dfrac{qrst}{pqrst}=\dfrac{248}{pqrst} \rightarrow\;\dfrac{1}{t}+\dfrac{1}{r}+\dfrac{1}{q}+\dfrac{1}{p}=\dfrac{248}{pqrst}-(4)$

$\dfrac{pqrt}{pqrst}+\dfrac{pqst}{pqrst}+\dfrac{prst}{pqrst}+\dfrac{qrst}{pqrst}=-\dfrac{16}{pqrst}\rightarrow\;\dfrac{1}{s}+\dfrac{1}{r}+\dfrac{1}{q}+\dfrac{1}{p}=-\dfrac{16}{pqrst}-(5)$

Equation (2) $-$ Equation (1):
$\dfrac{1}{p}-\dfrac{1}{r}=\dfrac{240}{pqrst}-(6)$

Equation (3) $-$ Equation (2):
$\dfrac{1}{r}-\dfrac{1}{q}=\dfrac{48}{pqrst}-(7)$

$\therefore \dfrac{1}{p}-\dfrac{1}{q}=\dfrac{288}{pqrst}--(*)$

Equation (4) $+$ Equation (5):
$\dfrac{1}{t}+\dfrac{1}{s}+2\left(\dfrac{1}{r}+ \dfrac{1}{q}+ \dfrac{1}{p} \right)=\dfrac{232}{pqrst}-(8)$

Equation (7) $-$ Equation (3):
$2\left(\dfrac{1}{q}\right)+ \dfrac{1}{r}+ \dfrac{1}{p}=\dfrac{208}{pqrst}-(9)$

Equation (8) $-$ Equation (6):
$3\left(\dfrac{1}{q}\right)+ \dfrac{1}{p}=\dfrac{160}{pqrst}-(**)$

Solving the equations (*) and (**) simultaneously we get:
$\dfrac{1}{q}=-\dfrac{32}{pqrst},\dfrac{1}{p}=\dfrac{256}{pqrst}, \dfrac{1}{r}=\dfrac{16}{pqrst}, \dfrac{1}{s}=-\dfrac{256}{pqrst}, \dfrac{1}{t}=\dfrac{8}{pqrst}$

Therefore, if we multiply these five equations together, we get
$\dfrac{1}{pqrst}=\dfrac{2^{28}}{(pqrst)^5}$

$\therefore pqrst=2^7$ or $\dfrac{1}{q}=-\dfrac{1}{4},\dfrac{1}{p}=2, \dfrac{1}{r}=\dfrac{1}{8}, \dfrac{1}{s}=-2, \dfrac{1}{t}=\dfrac{1}{16}$.

Hence $80t-20s+60r-50q+70p=80(16)-20(-\dfrac{1}{2})+60(8)-50(-4)+70(\dfrac{1}{2})=2005$.
 
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