JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
The number of integral values of k, for which one root of the equation lies in the interval (1, 2) and its other root lies in the interval (2, 3), is :
- A2
- B0
- C1
- D3
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Correct answer: C
-
Given quadratic
Let its roots be and with
-
Use sum and product of roots
For the equation ,
\qquad \alpha\beta=\frac{k}{2}.$$ -
Use the interval condition with the sum
Since , write Since ,
Now check whether lies in :
- If , then So the second condition is automatically satisfied.
Hence we only need where is a root and the other root is .
-
Express roots explicitly
Solve the quadratic:
Therefore,
So the two roots are
\qquad 2+\frac{\sqrt{16-2k}}{2}.$$ For one root to be in $(1,2)$ and the other in $(2,3)$, we need $$1<2-\frac{\sqrt{16-2k}}{2}<2$$ and automatically the other will be in $(2,3)$ by symmetry. -
Solve the inequality
From we get
Also, for the smaller root to be less than , we need so the roots are distinct and not both equal to .
Thus,
Squaring,
This gives and
Hence,
-
Integral values of
The only integer in is
Therefore, the number of integral values of is
-
Option check
- A: ❌
- B: ❌
- C: ✅
- D: ❌
Final Answer: , i.e. Option C.
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