First we identify the sides that are known or available. We have the Adjacent and Hypotenuse x is the adjacent. The Hypotenuse has been given with a value 13.
We must identify what function to use to find the unknown side x. The formulae we have are sine, cosine, or tangent.
We could use SOHCAHTOA to remember the full trigonometry formulae
In this example we must use the cosine formula because it involves the adjacent and the hypotenuse;
All we have to do is substitute in the known values to find the unknown values as shown below;
We must then rearrange to make x the subject since it is the value that we are after.
Now we need to work out the value of 13cos(40°) on the calculator and round off to 2d.p, the answer would be;