The 8 pound bag of dog food for $7.40 is the better buy.
The cost of another 2 panes of glass is 5.70
Explanation is given step by step just below:
tan(theta(t))=y(t)/x(t)
differentiate
sec^2(theta(t))*theta'(t)=y'(t)x(t)-y(t)x'(t)/x^2(t)
thus
theta'(t)=(y'(t)x(t)-y(t)x'(t))
divided by (x^2(t)*sec^2(θ(t))
