I am studying in IGCSE and I learned simple techniques to find, say, approximate change in Area of a circle for a small change in its radius, making use of :
δy/ δx ≈ dy/dx .
or δA/ δr ≈ dA/dr .
or δA ≈ dA/dr x δr
so what I basically have to do is find the derivative of A ( πr2 ) , which...