let centre of circle with radius 1 ft= A

let centre of circle with radius 2 ft= B

let centre of circle with radius 3 ft= c

join ABC

so AB=3 ft,BC=5ft,CA=4 ft

it clearly shows that ABC is a right angle triangle with angle CAB=90 degrees.

now we may calculate area of sector of circle with centre A

since angle A is 90 degrees therefor

area of sector = (1/4) area of circle

=(1/4)(pi (1)^2)

=pi/4 square ft