My undergrad senior-year Mechanical Vibrations book tells me that I should remember the notion that Asin(θ+ø) can also be represented in the form of Bsinθ+Ccosθ (and other linear combinations of sines and cosines), from high-school trigonometry class. However, I was never taught this in my high-school. I even read a lot of material and watched a lot of YouTube videos on trigonometry in my high-school years, but the notion that Asin(θ+ø) can also be represented in the form of Bsinθ+Ccosθ etc never came up. I am just wondering where exactly in a average high-school curriculum are we taught this notion? Answer of one or two sentences would be enough.