We have the following equation:
x2 + 4x = - (y2 - 20)
Let's review step by step:
step 1:
x2 + 4x = -y2 + 20
Answer: OK
step 2:
x2 + 4x + 4 = -y2 + 20 - 4
Answer: Wrong
The correct step is:
x2 + 4x - 4 = -y2 + 20 - 4
Or equivalently:
x2 + 4x + 4 = -y2 + 20 + 4
Answer:
he did make an error in his work in step 2
C^2 - d^2 / d^2 + cd - 2c^2
= cd + c^2 - 2c^2 - d^2 / d^2
= cd - c^2 - 1 (answer)
The answers for these questions are D and B
