5sin[(A + B) - A] = sin[(A + B) + A]左边 = 5sin(A + B)cosA - 5cos(A + B)sinA右边 = sin(A + B)cosA + cos(A + B)sinA ,移项得：4sin(A + B)cosA = 6cos(A + B)sinA两边除以 2cosAcos(A + B) 即得：2tan(A+B)=3t...