Question

A fence 6 ft high runs parallel to the wall of a house at a distance of 5 ft. Find the length of the shortest ladder that extends from the​ ground, over the​ fence, to the house. Assume the wall of the house is 25 ft high and the horizontal ground extends 20 ft from the fence.

Answer 1

Answer:

L = 15.53 ft

Step-by-step explanation:

using diagram which is attached below

now, using Pythagoras theorem

L² = b²  + (5 + x)²...............(1)

using similarity of triangle

$\dfrac{x}{6}=\dfrac{5+x}{b}$

$b=\dfrac{5+x}{x}6$

from equation (1)

$L^2 = (\dfrac{5+x}{x}6)^2 + (5 + x)^2$

     $= (5+x)^2(1+\dfrac{36}{x^2})$

$\frac{\mathrm{d} L^2}{\mathrm{d} x}= 2(5+x)(1+\dfrac{36}{x^2})+(5+x)^2(\dfrac{-2\times 36}{x^3})$

$\frac{\mathrm{d} L^2}{\mathrm{d} x}= (5+x)(2-\dfrac{72\times 5}{x^3})$

for maxima or minima

$\frac{\mathrm{d} L^2}{\mathrm{d} x}=0$

x = -5 and x =  ∛190  = 5.74

on second derivative 5.74 is the value which comes out to be minimal value which is the distance between the fence and ladder

now,

L² = 241.38

L = 15.53 ft