MAINT: Use trig identity to evaluate cosm1.

Author: tillahoffmann

numpy/core/src/umath/funcs.inc.src

@@ -360,8 +360,8 @@ nc_exp2@c@(@ctype@ *x, @ctype@ *r)
 static void
 nc_expm1@c@(@ctype@ *x, @ctype@ *r)
 {
-    @ftype@ a = npy_cos@c@(x->imag);
-    r->real = npy_expm1@c@(x->real) * a + (a - 1);
+    @ftype@ a = npy_sin@c@(x->imag / 2);
+    r->real = npy_expm1@c@(x->real) * npy_cos@c@(x->imag) - 2 * a * a;
     r->imag = npy_exp@c@(x->real) * npy_sin@c@(x->imag);
     return;
 }