Factorise $2\pi r^{2} + 2\pi r h$.

Factorise $2\pi r^{2} + 2\pi r h$.
  1. $2\pi r(r + h)$
  2. $2\pi(r^{2} + h)$
  3. $\pi r(2r + 2h)$
  4. $2\pi(r + h)$

Solution

Common factor is $2\pi r$. $2\pi r^{2} + 2\pi rh = 2\pi r(r + h)$ (this is the curved + base area of an open cylinder).

Asked in: IMO

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