What should be taken away from 3x² +4y²+5xy+20 to obtain -x² – y² +6xy+20?

Let q should be subtracted.
Then according to question,
3x²+4y²+5xy+20-q = -x² – y² +6xy+20?
⇒ q= 3x²+4y²+5xy+20-(-x² – y² +6xy+20)
⇒ q= 3x²+4y²+5xy+20 + x² + y² + 6xy-20
⇒ q= 3x² +x²-4y²+y²+5xy-6xy+20-20
⇒ q= 4x²-3y²-xy+0
Hence, 4x²-3y²-xy should be subtracted.

Class 7 Maths Chapter 12 Exercise 12.2

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https://www.tiwariacademy.com/ncert-solutions/class-7/maths/chapter-12/