Question

Subtract:

 a² − b² from b² − a²

Answer 1

Change the sign of the term of the expression that is to be subtracted and then add.

Term to be subtracted = a² − b²
Changing the sign of each term of the expression gives -a² + b².
On adding:
b² − a² + (-a² + b²) = b² − a² -a² + b²
= (1+1)b² +(−1-1) a²
= 2b² − 2a²