Hello Nusrat Nazir

I hope you are doing good.  I would glad to give answer to this query as calculus was my favourite topic at the time of entrance preparation

even multiple of pie are  nothing basically positive x axis

negative mutilple of pie are nothing bascially negative x axis

IN case of cos , cos is positive in 1st and 4th quadrant , so if you substract some angle from  even and odd multiple of pie you will always land positive values if cos

In case of sin, sin is positive in 1st and 2nd quad , so if you substract some angle from negative x axis and if add some angle in positive x axis , you will comes out with positive values of sin

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Thankyou