Prove that 3(x+1)(x+7) - (2x+5)² is never positive
So far, I have got:
(x+1)(x+7) = x²+8x+7
3(x²+8x+7)=3x²+24x+21
(2x+5)²=(2x+5)(2x+5)= 4x²+20x+25
(3x²+24x+21)-(4x+20x+25)=x²-4x-4
-x²-4x-4=-1(x²+4x+4)=-1((x+2)(x+2))
Because it will always be multiplied by -1, it is never positive
This is out of 5 marks but I only got 4 marks. I don't know where the remaining mark is........
So far, I have got:
(x+1)(x+7) = x²+8x+7
3(x²+8x+7)=3x²+24x+21
(2x+5)²=(2x+5)(2x+5)= 4x²+20x+25
(3x²+24x+21)-(4x+20x+25)=x²-4x-4
-x²-4x-4=-1(x²+4x+4)=-1((x+2)(x+2))
Because it will always be multiplied by -1, it is never positive
This is out of 5 marks but I only got 4 marks. I don't know where the remaining mark is........