Question 814281
  <pre><font face = "Tohoma" size = 3 color = "indigo"><b> 
Hi,
sin(a + b) = sin a cos b  +  cos a sin b
{{{sin(a+ pi/2)= sin( a) *cos (pi/2)  +  cos (a) *sin (pi/2) }}}= cos a
Note: {{{cos(pi/2) = 0}}} and  {{{sin(pi/2) = 1}}}