Asked by Anonymous

Prove 3(x+1)(x+7)-(2x+5)² is never positive

So,
3(x+1)(x+7)-(2x+5)(2x+5)
=3(x²+8x+7)-(4x²+20x+25)
=3x²+24x+21-4x²-20x-25
=-x²+4x-4
Answered by Steve
= -(x^2-4x+4)
= -(x-2)^2
Answered by Anonymous
The answer above didn't work
Answered by Anonymous
Sorry It actually did
Answered by Steve
I don't know what you mean by it "didn't work." It just shows that the expression is never positive.

(x-2)^2 is a square, so it is always positive

So, -(x-2)^2 is never positive.

Trying to plug a formula into a request for a proof will never work. The logic is what counts.
Answered by Anonymous
this was extremely useful. thank you
Answered by Anonymous
Hi, you information was really useful but for some reason I only got 4 marks out of 5 on mathswatch, do you know why?
Answered by your maeum
THANK YOU SM
Answered by Harini
3(x + 1)(x + 7) - (2x + 5)(2x + 5)
= 3(x² + 8x + 7)- (4x²+ 20x + 25)
= 3x² + 24x + 21- 4x²- 20x- 25
= -x² + 4x - 4
= -1(x - 2)²
A squared number is always positive, but when you multiply it with a negative number, it is always negative, never positive.

^This will give you all 5 marks on Mathswatch. Hope it helps :)
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