Question

Ex 15 pls dau coroana

Answer 1

Explicație pas cu pas:

15)

$\frac{1}{1\cdot 2} + \frac{1}{2 \cdot 3} + ... + \frac{1}{23 \cdot 24} + \frac{1}{24 \cdot 25} =$

$= \frac{1}{1} - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + ... + \frac{1}{23} - \frac{1}{24} + \frac{1}{24} - \frac{1}{25}$

$= \frac{1}{1} - \frac{1}{25} = \frac{25 - 1}{25} = \frac{24}{25}$

$\frac{1}{25 \times 26} + \frac{1}{26 \times 27} + ... + \frac{1}{48 \times 49} + \frac{1}{49 \times 50} =$

$= \frac{1}{25} - \frac{1}{26} + \frac{1}{26} - \frac{1}{27} + ... + \frac{1}{48} - \frac{1}{49} + \frac{1}{49} - \frac{1}{50}$

$= \frac{1}{25} - \frac{1}{50} = \frac{2 - 1}{50} = \frac{1}{50}$

=>

$x = \sqrt{ \frac{1}{94} \left( \frac{24}{25} - \frac{1}{50} \right)} = \sqrt{ \frac{1}{94} \times \frac{48 - 1}{50}} \\ = \sqrt{ \frac{1}{94} \times \frac{47}{50}} = \sqrt{ \frac{1}{100} } = \frac{1}{10}$

q.e.d.