Thread: help please with this problem

Originally Posted by jim49990

a cylindrical concrete pipe is to be cast so that the concrete is uniformally 75mm thick while the cross section of the concrete is 0.164m squared. formulate and solve the simultaneous equations showing the relationship between the external radius, R, and the internal radius r, of the pipe
many thanks
jim

Let R = external radius
And r = internal radius

Given:
R -r = 75mm = 0.075 m -----------(1)

pi(R^2 -r^2) = 0.164 sq.m. -------(2)

So,
pi[(R+r)(R-r)] = 0.164
pi[(R+r)(0.075)] = 0.164
R+r = 0.164 / 0.075pi
R +r = 0.696 m ---------------------(3)

(1) plus (3),
2R = 0.075 +0.696 = 0.771
R = 0.771 /2 = 0.3855 m -----------***

(3) minus (1),
2r = 0.696 -0.075 = 0.621
r = 0.621 /2 = 0.3105 m ------------***