A teacher gave her class two exams; 60% of the class passed the second exam, but only 48% of the class passed both exams. What p
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The equation of the line through (0, 1) and (<em>c</em>, 0) is
<em>y</em> - 0 = (0 - 1)/(<em>c</em> - 0) (<em>x</em> - <em>c</em>) ==> <em>y</em> = 1 - <em>x</em>/<em>c</em>
Let <em>L</em> denote the given lamina,
<em>L</em> = {(<em>x</em>, <em>y</em>) : 0 ≤ <em>x</em> ≤ <em>c</em> and 0 ≤ <em>y</em> ≤ 1 - <em>x</em>/<em>c</em>}
Then the center of mass of <em>L</em> is the point with coordinates given by
where is the first moment of <em>L</em> about the <em>x</em>-axis,
is the first moment about the <em>y</em>-axis, and <em>m</em> is the mass of <em>L</em>. We only care about the <em>y</em>-coordinate, of course.
Let <em>ρ</em> be the mass density of <em>L</em>. Then <em>L</em> has a mass of
Now we compute the first moment about the <em>y</em>-axis:
Then
but this clearly isn't independent of <em>c</em> ...
Maybe the <em>x</em>-coordinate was intended? Because we would have had
and we get