How to prove that sin (pi/2 – theta) = cos theta ?

see explanation
using appropriate ##color(blue) Addition formula ##
## sin(A B) = sinAcosB cosAsinB ##
hence ## sin(pi/2 -theta) = sin(pi/2) costheta – cos(pi/2)sintheta ##
now ## sin(pi/2) = 1 and cos(pi/2) = 0 ##
hence ## sin(pi/2)costheta – cos(pi/2)sintheta = costheta – 0 ##
##rArr sin(pi/2 – theta ) = costheta ##