Question

Use completing the square to determine which of the following gives the solutions to 3x2 - 24x + 40 = 28.

Answer 1

Answer:

$x=4+2\sqrt{3}$

$x=4-2\sqrt{3}$

Step-by-step explanation:

Completing the square

when  $ax^2+bx+c=0$

Given equation:

$3x^2-24x+40=28$

Subtract 40 from both sides:

$\implies 3x^2-24x=-12$

Divide both sides by 3:

$\implies x^2-8x=-4$

Add the square of half the coefficient of $x$ to both sides:

$(\frac{b}{2})^2=(\frac{-8}{2})^2=16$

$\implies x^2-8x+16=-4+16$

Factor the left side and simplify the right side:

$\implies (x-4)^2=12$

Square root both sides:

$\implies x-4=\pm\sqrt{12}$

Add 4 to both sides and simplify the radical:

$\implies x=4\pm\sqrt{4 \cdot 3}$

$\implies x=4\pm\sqrt{4}\sqrt{3}$

$\implies x=4\pm2\sqrt{3}$