当x<=2c时，f(x) = x + 2c-x -1 = 2c-1 > 0 =>c > 1/2当x > 2c时，f(x) = x + x -2c -1 = 2x-2c-1>0 => x > c+1/2 即x > 2c时 x > c+1/2 所以 2c>c+1/2 => c>1/2反之，当c >1/2时, 可推出f(x） > 0故充要条件为 c>1/2
