(related to Proposition: Special Values for Real Sine, Real Cosine and Complex Exponential Function)
For all real numbers $x\in\mathbb R$, the values of the real cosine and real sine are equal to each other for the arguments $x$ and $\pi/2-x,$ (where $\pi$ denotes the $\pi$ constant), formally $$\cos\left(\frac\pi2-x\right)=\sin(x),\quad\quad\sin\left(\frac\pi2-x\right)=\cos(x).$$
Proofs: 1