Posted by sarah on Saturday, April 17, 2010 at 12:19am.
An isosceles triangle has two 10.0inch sides and a 2winch side. Find the radius of the inscribed circle of this triangle, in the cases w = 5.00, w = 6.00, and w = 8.00.
Then Write an expression for the inscribed radius r in terms of the variable w , then find the value of w, to the nearest hundredth, that gives the maximum value of r.

geometry  Reiny, Saturday, April 17, 2010 at 8:46am
The inscribed circle has its centre on the bisectors of the angles.
Because of the properties of isosceles triangles that angle bisector also becomes the rightbisector of the nonequal side, or our 2w base.
I will do the w=8 case.
draw a 101016 triangle, 16 as the base
label the base angle 2Ø, thus each of the bisected base angles are Ø.
label the length of the radius on the rightbisector as r, (where the angle bisector meets the righbisector of 16)
cos 2Ø = 8/10 = 4/5
we know cos 2Ø = 2cos^ Ø  1
4/5 = 2cos^2 Ø 
cos^2Ø = (4/5 + 1)/2 = 9/10
cosØ = 3/√10
then sinØ = 1/√10 , using Pythagoras
back to the triangle, in the smaller rightangled triangle
tanØ = r/8
sinØ/cosØ = r/8
(1/√10)/(3/√10) = r/8
1/3 = r/8
r = 8/3
Now you repeat the calculations for w = 6 and see if you can see the pattern.
then generalize, or go through the same process, for w.

geometry Thanks Reiny  Sarah, Saturday, April 17, 2010 at 12:38pm
Thanks for the help.
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