cos(?-x)=-cosx.. if x=90 then cos(?-x) = cos 90 then how it is equal to - cos 90 ..plz explain..? Share with your friends Share 0 Brijendra Pal answered this Hi, cos 90 = -cos90 = 0 so it does not matter here BUT to get the idea about formula cosπ-x= cosπcosx+sinπsinx= -1×cosx+0×sinx= -cosx formula used cosa-b= cosacosb+sinasinb 1 View Full Answer Parul Awasthi answered this I think 0 Manav answered this Is it maths or physics. 0 Dipesh Kumar Karesia answered this in place of ? it is pi..... plz explain it os of maths ...... 0