cos(?-x)=-cosx.. if x=90 then cos(?-x) = cos 90 then how it is equal to - cos 90 ..plz explain..?

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

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I   think
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Is it maths or physics.
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in place of ? it is pi..... plz explain it os of maths ......
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