c2-c1,c3-c2,c4-c3a^2 2a+1 2a+3 2a+5b^2 2b+1 2b+3 2b+5c^2 2c+1 2c+3 2c+5d^2 2d+1 2d+3 2d+5c2+c4a^2 2a+1 2a+3 4a+6b^2 2b+1 2b+3 4b+6c^2 2c+1 2c+3 4c+6d^2 2d+1 2d+3 4d+6c4=2c3所以D=0
