Math Problem Statement

if x+y=8, log_2(xy)=2, then y=

Solution

We are given:

  1. x+y=8x + y = 8
  2. log2(xy)=2\log_2(xy) = 2

Step 1: Simplify the second equation

From log2(xy)=2\log_2(xy) = 2, we rewrite this in exponential form:

xy=22=4xy = 2^2 = 4

So, xy=4xy = 4.

Step 2: Solve the system of equations

We now have two equations:

  1. x+y=8x + y = 8
  2. xy=4xy = 4

Let xx and yy be the roots of a quadratic equation. A quadratic equation with roots xx and yy can be written as:

t2(x+y)t+xy=0t^2 - (x + y)t + xy = 0

Substitute x+y=8x + y = 8 and xy=4xy = 4:

t28t+4=0t^2 - 8t + 4 = 0

Step 3: Solve the quadratic equation

The quadratic equation is:

t28t+4=0t^2 - 8t + 4 = 0

Use the quadratic formula:

t=b±b24ac2at = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

Here, a=1a = 1, b=8b = -8, and c=4c = 4. Substitute these values:

t=(8)±(8)24(1)(4)2(1)t = \frac{-(-8) \pm \sqrt{(-8)^2 - 4(1)(4)}}{2(1)} t=8±64162t = \frac{8 \pm \sqrt{64 - 16}}{2} t=8±482t = \frac{8 \pm \sqrt{48}}{2} t=8±432t = \frac{8 \pm 4\sqrt{3}}{2} t=4±23t = 4 \pm 2\sqrt{3}

Step 4: Assign xx and yy

The roots are t1=4+23t_1 = 4 + 2\sqrt{3} and t2=423t_2 = 4 - 2\sqrt{3}. Since x+y=8x + y = 8, the values of xx and yy are:

x=4+23,y=423x = 4 + 2\sqrt{3}, \quad y = 4 - 2\sqrt{3}

or vice versa.

Thus, y=423y = 4 - 2\sqrt{3} (assuming y<xy < x).

Final Answer:

y=423y = 4 - 2\sqrt{3}

Would you like me to clarify any step further?


Related Questions:

  1. How do you simplify quadratic equations further when roots involve square roots?
  2. Can log2(xy)=2\log_2(xy) = 2 be solved for xx and yy without using the quadratic method?
  3. What if the logarithm had a different base? How would it change the process?
  4. How does the discriminant of a quadratic equation determine the nature of its roots?
  5. Can xx and yy ever be equal in such cases? Why or why not?

Tip: Always verify solutions by substituting back into the original equations!

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Math Problem Analysis

Mathematical Concepts

Algebra
Logarithms
Quadratic Equations

Formulas

Logarithmic formula: log_b(a) = c implies a = b^c
Quadratic equation formula: t^2 - (x + y)t + xy = 0
Quadratic formula: t = (-b ± √(b^2 - 4ac)) / 2a

Theorems

Properties of logarithms
Quadratic formula theorem

Suitable Grade Level

Grades 9-12