Accelerometer - alignment

I have been using an accelerometer and I have realised that it was sitting at a tilt of 18 degrees in the z direction and rotated at 5 degrees in the x y direction. How should I change my accelerometer readings to account for this?

 

So for example my z reading will be multiplied by cos(18) and cos(5) as will the x and y readings?

 

Just wanted to check, so I will divide all readings by the cosine of both angles. For example I will divide z readings by cosing 18 AND cosine5 .. It makes sense as both rotations will effect all vectors but wanted to be sure

 

Also wanted to check, Why do we use cosine as this would suggest that the actual reading and reading nw made form a right angle triangle with eachother?

 

How would you approach this problem A.T?

 

Hi there AT thank you so much for the advice. I actually came across my error I had already come up with a rotation matrix before but had considered the rotations in the wrong order. I also wanted to check how would work out the roll pitch and yaw from stationary data? my data is -0.9 0.5 and -0.4 (my x is pointing downwards into the 1g direction) How can I work out the angles I have done it but I think error in this part is leading to my problem

 

That is a great idea however, I am doing this in retrospect. The device was placed on an animal and rotated during the course of the experiment. As a result the only thing I have to go on is the x y z readings when the animal was stationary in order to work out how much the gyroscope had rotated and then correct the rest of the readings. So what I found was that at rest my readings were x = -0.9 y = 0.14 z = 0.44 and so I am trying to adjust the rest of my readings based on this info