If cosp+sinp=2cosp, then check that 2sinp=cospsinp.
 
Proof:
 
Consider cosp+sinp=2cosp.
 
Squaring on both sides, we get:
 
 
 
 
 
 
 
 
 
Hence, we proved.
Answer variants:
2cospsinp=2cos2pcos2psin2p
cos2p+sin2p+2cospsinp=2cos2p
2sinp=cospsinp
cosp+sinp2=2cosp2
2cospsinp=cosp+sinpcospsinp
2cospsinp2cosp=cospsinp
2cospsinp=2cospcospsinp
2cospsinp=cos2psin2p