Question 189213
25*35=875 is the total area.
875-375=500 is the area of the border.
(25-2x)(35-2x)=500
875-120x+4x^2=500
4x^2-120x+875-500
4x^2-120x+375
Using the quadratic equation {{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} we get:
x=(120+-sqrt[-120^2-4*4*375])/2*4
x=(120+-sqrt[14,400-6,000])/8
x=(120+-sqrt[8,400}/8
x=(120+-91.65)/8
x=(120-91.65)/8
x=28.35/8
x=3.54 ans for the width of the border.
Proof:
(25-2*3.54)(35-2*3.54)=500
(25-7.08)(35-7.08)=500
(17.92*27.92)=500
500=500