678 + 2 = 670 + (8 + 2) - 670 + 10 = 791 + 9 = 790 + (1 + 9) = 790 + 10 = 992 + 8 = 990 + (2 + 8) = 990 + 10 - 680-2=（670+10）-2=670+（10-2）=670＋8＝口 800 - 9 = (790 + 10) - 9 = 790 +…

Let's solve each of the mathematical expressions step by step.

1. For the first expression: \[ 678 + 2 = 670 + (8 + 2) - 670 + 10 \] Simplifying:

   • We can compute \(678 + 2 = 680\).
   • The right side becomes \(680 - 670 + 10 = 10 + 10 = 20\). So \( 678 + 2 \neq 670 + (8 + 2) - 670 + 10\).

2. For the second expression: \[ 791 + 9 = 790 + (1 + 9) \] Simplifying:

   • We can compute \(791 + 9 = 800\).
   • The right side becomes \(790 + 10 = 800\), which matches the left side. So both sides equal \(800\).

3. For the third expression: \[ 992 + 8 = 990 + (2 + 8) \] Simplifying:

   • We can compute \(992 + 8 = 1000\).
   • The right side computes to \(990 + 10 = 1000\), which matches the left side. So both sides equal \(1000\).

4. For the fourth expression: \[ 680 - 2 = (670 + 10) - 2 \] Simplifying:

   • We can compute \(680 - 2 = 678\).
   • The right side computes to \(670 + 10 - 2 = 678\), which matches the left side. So both sides equal \(678\).

5. For the fifth expression: \[ 800 - 9 = (790 + 10) - 9 \] Simplifying:

   • We can compute \(800 - 9 = 791\).
   • The right side computes to \(790 + 10 - 9 = 791\), which matches the left side. So both sides equal \(791\).

6. For the sixth expression: \[ 1000 - 8 = (990 + 10) - 8 \] Simplifying:

   • We can compute \(1000 - 8 = 992\).
   • The right side computes to \(990 + 10 - 8 = 992\), which matches the left side. So both sides equal \(992\).

To summarize:

• \( 678 + 2 = 680 \)
• \( 791 + 9 = 800 \)
• \( 992 + 8 = 1000 \)
• \( 680 - 2 = 678 \)
• \( 800 - 9 = 791 \)
• \( 1000 - 8 = 992 \)