# [C (gcc)], <sup><s>66</s> <s>59</s></sup> 54 bytes  

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    f(a,n,p,r)float*r,n;{for(*r=1;p--;)*r*=(a-p)/(a-p/n);}

[Try it online!][TIO-kn1whtxo]

[C (gcc)]: https://gcc.gnu.org/
[TIO-kn1whtxo]: https://tio.run/##fVLRjpswEHzPV6wipcKcaWwMBkTTl6pf0USVS8wVNTUWRmrUKL9eamOgyaV3aNdiZ8fDsHYVPVfVMNSBwApr3KH61Io@7LAqL3XbBWG3o6WOohKFXbgLRKTR1q1bhcrr8FM0KkCrywrs06geemn6r@LLAXZwiTH4SAkGdh8xf0wWz3kt7wWVF6QYlojfjNzHSx3tdcis4IwthXWV3JTUuXSVK12bTWrjgECetax6eXx05oO8z92yKI5QkRNWxDTlOY@zJB0xyhKaMcITlmYsyfNxGyG0oJxwi1NepCwuRmqWZQlhlPOcWph5JqEJZUWRcJskt@oOn7xW30UX2lVWP2Tnra7358/x/lx8spmuMdzWbH3/j9o0p1bZXVRG2dRpOwjcQBt1lGfbIuX0@gFM81u2dTCPBm0nIFyQEp6eRjYCf2fm4xFWabo7Y/9QLm3vRc0E9UBw@/Xc1q/sNzeAveugMGgM7wx6SZRWaTnc/35KGGM5tfhmAhNJZH98GtQ/nu4ssw7WmyMGnxB9hE0NG7NXdujWgBsi8i4MXo7IiR8mT9fVdfhT1SfxbIbo118 "C (gcc) – Try It Online"    

Calculates the given formula. Returns a `float` through its last parameter (which is a pointer to an uninitialized float).