[" 185.If "y tan(A+B+C)=x tan(A+B-C)=lambda" ,then "tan2C=],[[" (a) "(lambda(x+y))/(lambda^(2)-xy)," (b) "(lambda(x+y))/(lambda^(2)+xy)],[" (c) "(lambda(x-y))/(xy-lambda^(2))," (d) "(lambda(x-y))/(xy+lambda^(2))]]