Question 850538
Given:  Rectangle, perimeter P;
x is length
y is width
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x=-1+4y, 
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UNKNOWNS: x and y.


IMPORTANT FACT
Perimeter for a rectangle, 2L+2W.


The current exercise in this solution uses x instead of L and y instead of W.
We are given P already.


{{{highlight(2x+2y=P)}}}
and {{{highlight(x=4y-1)}}}.
Solve for x and y.  Substitution would be a good way.
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{{{2x+2y=2(4y-1)+2y=P}}}
{{{8y-2+2y=P}}}
{{{10y-2=P}}}
{{{10y=P+2}}}
{{{highlight(y=(P+2)/10)}}}
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Use the formula for y to make and simplify the formula for x.
{{{x=4(substituteHere)-1}}}
{{{x=4((P+2)/10)-1}}}
That might be good as is, or...
{{{(2/5)(P+2)-1}}}
{{{(2/5)(P+2)-5/5}}}
{{{(2(P+2)-5)/5}}}
{{{(2P+4-5)/5}}}
{{{x=highlight(highlight((2P-1)/5))}}}----Yes, this form is better.
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SUMMARY:  Length, {{{highlight(x=(2P-1)/5)}}}, and width, {{{highlight(y=(P+2)/10)}}}