In trigonometry, particularly when analyzing right-angled triangles, each side of the triangle is assigned specific names based on a chosen angle. The adjacent side is the one that is next to the angle of interest and forms that angle together with the hypotenuse, which is the longest side of the triangle. It is essential to understand that the adjacent side is not the hypotenuse itself but rather the other side that touches the angle.

For instance, consider a right-angled triangle where you focus on one of the non-right angles. The side that is directly next to this angle (excluding the hypotenuse) is referred to as the adjacent side. Conversely, the side that is opposite this angle is called the opposite side. Recognizing which side is the adjacent side is crucial for utilizing trigonometric ratios, such as cosine, which is defined as the ratio of the length of the adjacent side to the length of the hypotenuse:

Being able to accurately identify the adjacent side is vital in solving various problems involving right-angled triangles, including determining missing side lengths or angles. This concept is fundamental to trigonometry, and mastering the identification and application of the adjacent side is essential for success in GCSE Maths.