tan 85 = tan(45 + 40)

=> tan 85 = (tan45 + tan40)/(1 - tan45 * tan40)

=> tan 85 = (1 + tan40)/(1 - 1 * tan40)             {since tan45 = 1}

 => tan 85 = (1 + tan40)/(1 - tan40)

So, tan 85 = tan45 ± tan40 or tan 85 = tan45 * tan40 is not possible.