Posted by Riley on Wednesday, April 22, 2009 at 10:44am.
Find an equation of the line through the point (3,5) that cuts off the least area from the first quadrant.
How do I solve? I am stuck. The only thing I can come with is this:
How to use this I don't know.
- Calculus - Reiny, Wednesday, April 22, 2009 at 11:37am
draw a line through (3,5) cutting the positive x and y axes at (a,0) and (0,b)
then the area of the triangle is
A = ab/2
but the slope of the two segments must be equal, so 5/(3-a) = (b-5)/-3
solving for b gave me b = 5a/(a-3)
then A = 5a^2/(2a-6)
using the quotient rule
dA/da = [(2a-6)(10a) - 5a^2(2)]/(2a-6)^2
= 0 for a max/min of A
simplifying the top and setting it equal to zero gave me
a^2 - 6a = 0
a = 0 or a = 6
clearly a=0 does not give me a triangle, so a = 6 and b= 10
so slope = 5/-3
and your equation would be
y-5 = (-5/3)(x-3)
take it from there.
- Calculus - Riley, Wednesday, April 22, 2009 at 11:46am
how did you get the slope of the two lines? I get everything else though.
- Calculus - Reiny, Wednesday, April 22, 2009 at 12:02pm
there is only one line
you were given the point (3,5) and I found
the x-intercept to be (6,0)
so slope = (5-0)/(3-6)
= 5/-3 = - 5/3
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