Question

Tính giá trị của các biểu thức:

A = \(\dfrac{1}{\sqrt{3}+\sqrt{5}}+\dfrac{1}{\sqrt{5}+\sqrt{7}}+\dfrac{1}{\sqrt{7}+\sqrt{9}}+...+\dfrac{1}{\sqrt{97}+\sqrt{99}}\)

B = 35 + 335 + 3335 + ... + 3333...(99 số 3)35

Answer 1

A = \(\dfrac{1}{\sqrt{3}+\sqrt{5}}+\dfrac{1}{\sqrt{5}+\sqrt{7}}+\dfrac{1}{\sqrt{7}+\sqrt{9}}+...+\dfrac{1}{\sqrt{97}+\sqrt{99}}\)

= \(\dfrac{1}{2}\left(\sqrt{5}-\sqrt{3}+\sqrt{7}-\sqrt{5}+\sqrt{9}-\sqrt{7}+...+\sqrt{99}-\sqrt{97}\right)\)

= \(\dfrac{1}{2}\left(\sqrt{99}-\sqrt{3}\right)\)

B = 35 + 335 + 3335 + ... + 3333...(99 số 3)35

= 33 + 2 + 333 + 2 + 3333 + 2 + ... + 333...33 + 2

= 2 . 99 + (33 + 333 + 3333 + ... + 333...3)

= 198 + \(\dfrac{1}{3}\)(99 + 999 + 9999 + ... + 999...99)

= 198 + \(\dfrac{1}{3}\)(10² - 1 + 10³ - 1 + 10⁴ - 1 + ... + 10¹⁰⁰ - 1)

= \(\left(\dfrac{10^{101}-10^2}{27}\right)+165\)