The value of λ for which the sum of squares of the roots of the quadratic equation x² + (3 − λ)x + 2 = λ has the least value, is:
Step-by-Step Solution
Key Concept: Sum of squares of roots = (sum)² - 2(product); minimize quadratic in λ
Step 1: Identify the roots and apply Vieta's formulas.
Let the roots of the quadratic equation $x^2 + (3-\lambda)x + 2 = \lambda$ be $\alpha$ and $\beta$.
Rewriting the equation in standard form:
$$x^2 + (3-\lambda)x + (2-\lambda) = 0$$
By Vieta's formulas:
- Sum of roots: $\alpha + \beta = -(3-\lambda) = \lambda - 3$
- Product of roots: $\alpha\beta = 2 - \lambda$
Step 2: Express the sum of squares of roots in terms of λ.
We use the algebraic identity:
$$\alpha^2 + \beta^2 = (\alpha + \beta)^2 - 2\alpha\beta$$
Substituting the values from Step 1:
$$\alpha^2 + \beta^2 = (\lambda - 3)^2 - 2(2 - \lambda)$$
Expanding:
$$\alpha^2 + \beta^2 = \lambda^2 - 6\lambda + 9 - 4 + 2\lambda$$
$$\alpha^2 + \beta^2 = \lambda^2 - 4\lambda + 5$$
Step 3: Find the value of λ that minimizes the sum of squares.
To find the minimum value, we take the derivative with respect to $\lambda$ and set it equal to zero:
$$\frac{d}{d\lambda}(\lambda^2 - 4\lambda + 5) = 2\lambda - 4 = 0$$
Solving for $\lambda$:
$$2\lambda = 4$$
$$\lambda = 2$$
Step 4: Verify this is a minimum.
The second derivative is:
$$\frac{d^2}{d\lambda^2}(\lambda^2 - 4\lambda + 5) = 2 > 0$$
Since the second derivative is positive, $\lambda = 2$ gives a minimum value.
**Final Answer:** The value of $\lambda$ for which the sum of squares of the roots is least is $\lambda = 2$.
This corresponds to **Option 2**.
Correct Answer: 3