## Problem 15 – Solution

**The problem:**

### Given:

### and

### Show that the identity:

### is true.

Admission exam to Engineering, Polytechnic Institute of Bucharest, Romania, 19xx Our Translation.#### We did not receive any good solution therefore we post

**Our Solution:**

### Let’s start by replacing and in , with the given values:

### And:

### [1]

### Using the formula:

### we have:

### a)

### b)

### c)

### d)

### but therefore

### and

### By replacing the expressions from a), b), c) and d) into [1] we have:

### =

### And:

### [2]

### Using the formula:

### the expression in [2] becomes:

## That’s means that:

## is true.

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