3sinα+2cosα/7sinα-4cosα= (-1/2)·(3sinα+2cosα/7sinα-4cosα)·(-2)= (-1/2)·{1 - (13sinα/[7sinα-4cosα])}设K = 13sinα/[7sinα-4cosα]= 13(sinα)^2/[7(sinα)^2 - 2sin2α]而(sinα)^2 = (tanα)^...