Answer:
r = 31.8
Step-by-step explanation:
To start with, we make the following assumptions. Let
r be the radius of semicircular end
l be the length of straight section
a be the area inside track, then
p = 2πr + 2l = 400
a = πr² + 2rl
2πr + 2l = 400
l = 200 - πr, substituting for l in area,
a = πr² + 2r(200 - πr)
a = πr² + 400r - 2πr²
a = 400r - πr²
In order to find our minimum area, we differentiate with respect to r and then it equate to zero
da/dr = 400 - 2πr = 0
r = 200/π, substituting for r in equation for l,
l = 200 - πr
l = 0 m
And since this is not possible, l must be 100 m (the minimum), and
r = (200 - 100)/π
r = 100/π = 31.8 m