# Thread: Area of stain

1. ## Area of stain

A drop of ink dropped onto a piece of cloth creates a circular stain.The radius of the stain increases at a constant rate of 3mm per second.

Determine how fast the area of stain is increasing with respect to time elapsed at the point in time when the radius of stain is 6mm.

2. Related rates nearly always depend on the chain rule, so you might want to try filling out this pattern...

... where straight continuous lines differentiate downwards (integrate up) with respect to the main variable (in this case time), and the straight dashed line similarly but with respect to the dashed balloon expression (the inner function of the composite which is subject to the chain rule).

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Don't integrate - balloontegrate!

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