I’m deriving the coefficients of a digital filter and I arrived to the expression

$$frac{2}{(n-57)pi}(sin((n-57)pi)-sin((n-57)pi/2))$$

while the next step is supposed to arrive to

$$frac{2}{(n-57)pi}(cos(npi/2))$$

Can someone please explain me what trigonometric identity am I supposed to apply here?