OG详解-OG5 数学2 Q7

The correct answer is either -30 or 30. Adding 7 to each side of the given equation yields (d-30)(d+30) = 0. Since a product of two factors is equal to 0 if and only if at least one of the factors is 0, either d-30 = 0 or d+30 = 0. Adding 30 to each side of the equation d-30 = 0 yields d = 30.  Subtracting 30 from each side of the equation d+30 = 0 yields d =-30. Therefore, the solutions to the given equation are -30 and 30. Note that -30 and 30 are examples of ways to enter a correct answer.