Want to know why this is wrong and how to do it (1 Viewer)

[VictorTango-9W]

Any help ASAP appreciated trials In 3 days i.e Monday and still this question bothers me tried it few days ago and now again still don't know why I'm wrong

 

[cossine]

The equal sign at (a) is incorrect.The trigonometric addition/subtraction angle identity has not been applied properly. Have another look at them again.

 

[VictorTango-9W]

The equal sign at (a) is incorrect.The trigonometric addition/subtraction angle identity has not been applied properly. Have another look at them again.

Yh lol I realised that just after I posted was thinking bout cos and how its cosxcosy for cos one and did that in here lol thx tho appreciate It