How can I apply trigonometry rules to an isosceles triangle?

First, split the isosceles triangle down the middle to create a right angled triangle. Then label the opposite, hypotenuse and adjacent. The hypotenuse will be the longest side of the triangle, the opposite will be opposite to any angle being given or looked for. Then the adjacent will be next to the angle given or being looked for. Then depending on what the question is looking for you will use the appropriate rule. For example, if two sides are involved and it is the opposite and hypotenuse, the sine (SOH) rule will be used to find the value of the angle (SOH).