My problems #5

Algebra Level 3

Solve for x x

3 x 2 + 6 x + 7 + 5 x 2 + 10 x + 14 = 4 2 x x 2 \sqrt{3x^2+6x+7}+\sqrt{5x^2+10x+14} = 4-2x-x^2


The answer is -1.

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1 solution

The equation can be rewritten as:

3 ( x + 1 ) 2 + 4 + 5 ( x + 1 ) 2 + 9 = ( x + 1 ) 2 + 5 \sqrt {3(x+1)^2+4} + \sqrt {5(x+1)^2+9}=-(x+1)^2+5

We have: 3 ( x + 1 ) 2 + 4 2 \sqrt {3(x+1)^2+4} \ge 2 and 5 ( x + 1 ) 2 + 9 3 \sqrt {5(x+1)^2+9} \ge 3 , so L H S 5 LHS \ge 5 for all real x x

Similarly, R H S = ( x + 1 ) 2 + 5 5 RHS = -(x+1)^2 +5 \le 5 .

For the equation to occur, x + 1 = 0 x+1 = 0 , or x = 1 x = -1

I am not able to understand how you have made rhs equation. Can you please again explain it.

Prakhar Gupta - 6 years, 1 month ago

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