B=2cosx-3cos(pi-x)+5sin((7pi)/2-x)+cos((3pi)/2-x)
=2cosx+3cos(-x)+5sin[3pi+(pi/2-x)]+cos[pi+(pi/2-x)]
=2cosx+3cosx-5sin(pi/2-x)-cos(pi/2-x)
=5cosx-5cosx-sinx
=-sinx
"-----------------------------------------------"
Công thức đã dùng:
*** cos(-x)=cosx
*** cos[(2k+1)pi+x]=-cosx (k in ZZ)
*** sin[(2k+1)pi+x]=-sinx (k in ZZ)
*** cos(pi/2-x)=sinx
*** sin(pi/2-x)=cosx