Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
If $$\alpha$$ and $$\beta$$ are roots of the equation, $$x^2 - 4\sqrt{2}kx + 2e^{4\ln k} - 1 = 0$$ for some $$k$$, and $$\alpha^2 + \beta^2 = 66$$, then $$\alpha^3 + \beta^3$$ is equal to:
We are given the equation $$x^2 - 4\sqrt{2}\,k\,x + 2e^{4\ln k} - 1 = 0$$ with roots $$\alpha$$ and $$\beta$$, and the condition $$\alpha^2 + \beta^2 = 66$$.
Step 1: Simplify the constant term.
Using the logarithm-exponential property: $$e^{4\ln k} = e^{\ln k^4} = k^4$$
So the equation becomes: $$x^2 - 4\sqrt{2}\,k\,x + 2k^4 - 1 = 0$$ $$-(*)$$
Step 2: Apply Vieta's formulas.
For the quadratic $$(*$$), by Vieta's formulas:
Sum of roots: $$\alpha + \beta = 4\sqrt{2}\,k$$ $$-(1)$$
Product of roots: $$\alpha \beta = 2k^4 - 1$$ $$-(2)$$
Step 3: Use the condition $$\alpha^2 + \beta^2 = 66$$.
We use the identity: $$\alpha^2 + \beta^2 = (\alpha + \beta)^2 - 2\alpha\beta$$
Substituting from $$(1)$$ and $$(2)$$:
$$66 = (4\sqrt{2}\,k)^2 - 2(2k^4 - 1)$$
$$66 = 32k^2 - 4k^4 + 2$$
$$4k^4 - 32k^2 + 64 = 0$$
Dividing by 4:
$$k^4 - 8k^2 + 16 = 0$$
Step 4: Solve for $$k$$.
Let $$u = k^2$$. Then $$u^2 - 8u + 16 = 0$$, which factors as $$(u - 4)^2 = 0$$.
So $$u = 4$$, meaning $$k^2 = 4$$, giving $$k = 2$$ or $$k = -2$$.
Note: While $$\ln k$$ requires $$k > 0$$ in the real number system, the expression $$e^{4\ln k} = k^4$$ is valid for all $$k \neq 0$$, and the problem states "for some $$k$$." We check both values.
Step 5: Compute $$\alpha + \beta$$ and $$\alpha\beta$$ for each $$k$$.
For $$k = 2$$: $$\alpha + \beta = 4\sqrt{2} \cdot 2 = 8\sqrt{2}$$, $$\alpha\beta = 2(16) - 1 = 31$$
For $$k = -2$$: $$\alpha + \beta = 4\sqrt{2} \cdot (-2) = -8\sqrt{2}$$, $$\alpha\beta = 2(16) - 1 = 31$$
Step 6: Compute $$\alpha^3 + \beta^3$$ using the identity.
$$\alpha^3 + \beta^3 = (\alpha + \beta)(\alpha^2 + \beta^2 - \alpha\beta) = (\alpha + \beta)(66 - 31) = 35(\alpha + \beta)$$
For $$k = 2$$: $$\alpha^3 + \beta^3 = 35 \times 8\sqrt{2} = 280\sqrt{2}$$
For $$k = -2$$: $$\alpha^3 + \beta^3 = 35 \times (-8\sqrt{2}) = -280\sqrt{2}$$
Step 7: Identify the answer from the options.
The value $$280\sqrt{2}$$ corresponds to Option B, and $$-280\sqrt{2}$$ corresponds to Option D. Since both are among the options, we select $$k = -2$$ which gives $$-280\sqrt{2}$$, matching Option D.
The correct answer is Option D: $$-280\sqrt{2}$$.
Create a FREE account and get:
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.