Real-valued solutions of exponential equation

CAT 2020 Slot 1 · QA · Hard · Inequalities

The number of real-valued solutions of the equation 2x+2x=2(x2)22^x + 2^{-x} = 2 - (x - 2)^2 is

  1. A.

    infinite

  2. B.

    0

  3. C.

    1

  4. D.

    2

Answer

B

Explanation

Let us analyze the Left Hand Side (LHS) and Right Hand Side (RHS):

  1. LHS: 2x+2x2^x + 2^{-x}. By AM-GM inequality, 2x+2x22x2x=22^x + 2^{-x} \ge 2\sqrt{2^x \cdot 2^{-x}} = 2. Equality holds iff 2x=2x    x=02^x = 2^{-x} \implies x = 0.

  2. RHS: 2(x2)22 - (x - 2)^2. Since (x2)20(x - 2)^2 \ge 0, we have 2(x2)222 - (x - 2)^2 \le 2. Equality holds iff x=2x = 2.

For LHS to equal RHS, both must equal 2 simultaneously. However, LHS =2= 2 at x=0x = 0, while RHS =2= 2 at x=2x = 2.

At x=0x = 0: LHS =2= 2, RHS =24=2= 2 - 4 = -2. At x=2x = 2: LHS =4.25= 4.25, RHS =2= 2.

Since there is no value of xx where LHS and RHS are equal, the number of real solutions is 0.

Practise this under exam conditions

Sign in to solve it with a live timer, the on-screen CAT calculator, and streak and accuracy tracking across every question you attempt.

Solve in the workspace