In the balanced chemical reaction,IO3- + aI- + bH+ →  CH2O + dI2a, b, c and d respectively correspond to

IO3- + aI- + bH+ →  CH2O + dI2I-1 →  I2 oxidation2I-1 →  I2 2I-1 →  I2 +2e----(1)IO3- →  I2 reduction2IO3-→I22IO3- + 12H+→  I2 + 6H2O2IO3- + 12H+ +10e-→  I2 + 6H2O ---(2)        (2) ⇒ 2IO3-+10e-+12H+→ I2 + 6H2O (1)×5⇒                        10I-→ 5I2 +10e-                 On addition: 2IO3-+10I-+12H+→ 6I2 + 6H2O                        IO3-+5I-+6H+→3I2+3H2OOn comparing, a = 5, b = 6, c = 3, d = 3