Question

What must be subtracted from a² + b² + lab to get – 4ab + 2b²? 

Answer 1

We get, the required result by subtracting

– 4ab + 2b² from a² + b² + 2ab

= a² + b² + 2ab – (– 4ab + 2b²)

= a² + b² + 2ab + 4ab – 2b² 

= a² + b² – 2b² + 2ab + 4ab

= a² – b² + 6ab