For a jet aircraft cruising in steady level flight with parabolic drag polar $C_D = C_{D0} + K C_L^2$, the condition for maximum Breguet range occurs when the induced drag coefficient $C_{Di}$ is related to zero-lift drag coefficient $C_{D0}$ as:

(A) C_{Di} = \frac{1}{3} C_{D0}

(B) C_{Di} = C_{D0}

(C) C_{Di} = 3 C_{D0}

(D) C_{Di} = 2 C_{D0}

Answer Key:

A

### 5-Stage Derivation & Analytical Solution: 1. **Stage 1: Breguet Equation:** $R = \frac{V}{c_t} \frac{C_L}{C_D} \ln(W_0/W_1) \propto \frac{C_L^{1/2}}{C_D}$. 2. **Stage 2: Maximization:** Differentiating $\frac{C_L^{1/2}}{C_{D0} + K C_L^2}$ with respect to $C_L$ and setting to zero yields $K C_L^2 = \frac{1}{3} C_{D0}$. 3. **Stage 3: Physical Meaning:** Induced drag $C_{Di} = K C_L^2 = \frac{1}{3} C_{D0}$. 4. **Stage 4: Selection:** Option A is correct.

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