(9x+9)^2=49

Question

How do i do this quadratic equation with the quadratic formula?

Answer 1

You don't actually need the quadratic formula for this one. 

 

This can be simplified by realizing that both of these are squares and by realizing that 49 is a perfect square. 

(9x+9)² = 7²

 

Take the square root of both sides and solve. Remember that you'd have to solve for both 9x+9 = 7 and 9x+9 = -7. 

 

If you really needed to use a quadratic, you'd do the following:

 

1. Expand to get 81x² + 162x + 81 = 49

2. Subtract the 49 from both sides to get 81x²+162x+32 = 0

3. A = 81, B = 162, C = 32

 

(-162 ± √(162)²-4(81)(32)) / 2(81)

 

Which gives you two cases: (-162 + √(15876))/162  and  (-162 - √(15876))/162

 

This gives you  (-162 + 126)/162 and (-162 - 126)/162 which end up being -2/9 and -16/9

 

As you can see, you have to deal with some pretty big numbers that can't be simplified nicely... which probably means that your teacher intended for you to do it the first way :)

 

Hope this helps!