Question 613947
{{{cos(x+pi)}}}
Just use the cos(A+B) formula:
cos(A+B) = cos(A)cos(B) - sin(A)sin(B)
with "A" being "x" and "B" being "{{{pi}}}":
{{{cos(x+pi) = cos(x)cos(pi) - sin(x)sin(pi)}}}
Since {{{cos(pi) = -1}}} and {{{sin(pi) = 0}}} this simplifies:
{{{cos(x+pi) = cos(x)cos(pi) - sin(x)sin(pi) = cos(x)*(-1) - sin(x)*0 = -cos(x)}}}