Question

cos45cos30-sin45sin30

Answer 1

cos(a+b) = cos(a)cos(b) - sin(a) sin(b)
Do you know that formula?
Or you can do it directly?

Answer 2

\[
\cos(45) =sin(45)= \frac {\sqrt 2}2\\
\sin(30)=\frac 1 2\\

\cos(30)= \frac {\sqrt 3}{2}

\]

Answer 3

Replace and you are done