-9 should be the answer <span />
I'm sure there's an easier way of solving it than the way I did, but I'm not sure what it could be. Never dealt with a problem like this before.
Anyway, I just plugged in and tested. Chose random values for a, b, c, and d, which follow the rule 0 < a < b < c < d:
a = 1
b = 2
c = 3
d = 4


Simplify into standard form:



Use the quadratic formula to solve:

For functions in the form of

. So in this case:
a = 1
b = -4
c = 2
Plug them in:

Solve for 'x':




So the answer would be A.
Let x represent the number of liters of 50% acid Theresa puts into the mix. The the number of liters of 30% acid will be (420-x). The total amount of acid in the final solution will be ...
0.50x + 0.30(420-x) = 0.45(420)
0.20x + 126 = 189 . . . . . . . . . . . . . . . simplify
0.20x = 63 . . . . . . . . . . . . . . . . . . . . . subtract 126
x = 63/0.20 = 315 . . . . . . . . . . . . . . . liters of 50% solution
(420-x) = 420-315 = 105 . . . . . . . . . liters of 30% solution
Theresa should mix ...
105 liters of 30% solution
315 liters of 50% solution
We have two unknowns: x and y. Now, we have to formulate 2 equations. The first would come from the use of the given ratio:
We use the distance formula to find the distance between coordinates:
3/4 = √[(x-4)²+(y-1)²] / √[(4-12)²+(1-5)²]
√[(x-4)²+(y-1)²] = 3√5
(x-4)²+(y-1)² = 45
x² - 8x + 16 + y² - 2y + 1 = 45
x² - 8x + y² - 2y = 28 --> eqn 1
The second equation must come from the equation of a line:
y = mx +b
m = (5-1)/(12-4) = 1/2
Substitute y=5 and x=12 for point (12,5)
5 = (1/2)(12) + b
b = -1
So, the second equation is
y = 1/2x -1 or x = 2 + 2y --> eqn 2
Solving the equations simultaneously:
(2 + 2y)² - 8(2 + 2y) + y² - 2y = 28
Solving for y,
y = -2
x = 2+2(-2) = -2
Therefore, the coordinates of point A is (-2,-2).