# Optimization problem - a question implying the solving process

• Apr 7th 2013, 10:35 AM
dokrbb
Optimization problem - a question implying the solving process
I found this problem with the provided solution.
I already solved such type of problem, but using the triangle similarities and applying the Pythagorean theorem. This solution, however, is shorter. The only thing I can't get is how have got this (see in Bold below). Can someone elaborate a little bit more steps out there(on how we get from what equals tan A to the value of A in degrees)?

Thanks,

A fence 6 feet tall runs parallel to a tall building at a distance of 5 feet from the building. What is the length of the shortest ladder that will reach from the ground over the fence to the wall of the building?

Let:
A = the angle the ladder makes with the ground
L = length of the ladder

L = 6/sinA + 5/cosA

dL/dA = -6cos/sin^2A + 5sinA/cos^2A = 0

-6cos^3A + 5sin^3A = 0

sinA/cosA = tanA = (6/5)^(1/3) = 1.06266

A = 46.73997 degrees

6/sinA = 8.23892

5/cos A = 7.29596

L = 8.23892 + 7.29596 = 15.53488 F
• Apr 8th 2013, 05:12 AM
dokrbb
Re: Optimization problem - a question implying the solving process
sinA/cosA = tanA = (6/5)^(1/3) = 1.06266

A = 46.73997 degrees

can you help please , it's not actually that I don't know how to calculate (maybe it's this, too.. :)) but it's because I get A = 60 something degrees and not 46+