Question

Can someone help me with a question (Euler's Formula)

Answer 1

I have
\[c_2(\cos(-t)-i \sin(-t))+c_3(\cos t-i \sin t)\]

which is
\[c_2(\cos t +i \sin t )+c_3(\cos t-i \sin t)\]

but how does this become

\[c_2\cos t + c_3\sin t\]