The initial object position is A(x1,y1,z1). We need rotate it into A′(x2,y2,z2) about z-axis by angle θ.And we take P be the any point in z-axis. It makes ∠APS=ϕ.
Now we find cosϕ=PS/OP=x1/r⇒x1=rcosϕ. But we know OQ=x1 but here slides take PS=x1 and match the answer.
My question is slides has wrong or my concepts is wrong? If both are wrong so how we find 3D rotation about any coordinate axis?