由和差化积公式:sinx+siny=2sin[(x+y)/2]cos[(x-y)/2]=1/3cosx+cosy=2cos[(x+y)/2]cos[(x-y)/2]=1/2两式相除：tg[(x+y)/2]=2/3所以tg(x+y)由2倍角公式即可求得.tan(x+y)=2tan[(x+y)/2]/{1-tan^2[(x+y)/2]=} =2*2/3/(...