[KizPer]

1.ให้ Sn=${\sum\limits_{k = 1}^{n} \frac{1 }{ \sqrt{k+1} k+ \sqrt{k}(k+1) } } $
จงหาค่าของ ${\lim_{n \to \infty}Sn} $


$\frac{1 }{ \sqrt{k+1} k+ \sqrt{k}(k+1) } = \frac{1}{\sqrt{(k+1)(k)}(\sqrt{k} + \sqrt{k+1})}$
=$\frac{(\sqrt{k} - \sqrt{k+1})}{\sqrt{(k+1)(k)}}(-1)$
=$\frac{1}{\sqrt{k}} - \frac{1}{\sqrt{k+1}}$

ตอบ 1

8.หาได้คำตอบไม่ค่อยสวย = $(b^2 - a^2)(ab)$