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Suppose kk is any integer such that the equation 2x2+kx+5=02 x^{2}+k x+5=0 has no real roots and the equation x2+(k−5)x+1=0x^{2}+(k-5) x+1=0 has two distinct real roots for xx. Then, the number of possible values of kk is

Solution

✅ Correct Option: 2

We need to find integer values of kk such that:

  1. 2x2+kx+5=02x^2 + kx + 5 = 0 has no real roots
  2. x2+(k−5)x+1=0x^2 + (k-5)x + 1 = 0 has two distinct real roots

For any quadratic equation ax2+bx+c=0ax^2 + bx + c = 0, the discriminant D=b2−4acD = b^2 - 4ac tells us about the nature of roots:

If D<0D < 0: No real roots

If D=0D = 0: One repeated real root

If D>0D > 0: Two distinct real roots


For 2x2+kx+5=02x^2 + kx + 5 = 0:

Here, a=2a = 2, b=kb = k, c=5c = 5

Discriminant: D1=k2−4(2)(5)=k2−40D_1 = k^2 - 4(2)(5) = k^2 - 40

For no real roots: D1<0D_1 < 0

k2−40<0k^2 - 40 < 0

k2<40k^2 < 40

Since 40=210≈6.32\sqrt{40} = 2\sqrt{10} \approx 6.32, we get:

−6.32<k<6.32-6.32 < k < 6.32

Since kk must be an integer: k∈{−6,−5,−4,−3,−2,−1,0,1,2,3,4,5,6}k \in \{-6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6\}


For x2+(k−5)x+1=0x^2 + (k-5)x + 1 = 0:

Here, a=1a = 1, b=(k−5)b = (k-5), c=1c = 1

Discriminant: D2=(k−5)2−4(1)(1)=(k−5)2−4D_2 = (k-5)^2 - 4(1)(1) = (k-5)^2 - 4

For two distinct real roots: D2>0D_2 > 0

(k−5)2−4>0(k-5)^2 - 4 > 0

(k−5)2>4(k-5)^2 > 4

When (k−5)2>4(k-5)^2 > 4, we have:

∣k−5∣>2|k-5| > 2

This absolute value inequality gives us two cases:

Case 1: k−5>2→k>7k-5 > 2 \rightarrow k > 7

Case 2: k−5<−2→k<3k-5 < -2 \rightarrow k < 3

So from condition 2: k<3k < 3 or k>7k > 7


We need values of kk that satisfy both conditions:

Condition 1: −6≤k≤6-6 \leq k \leq 6 (integers)

Condition 2: k<3k < 3 or k>7k > 7

The range k>7k > 7 doesn't overlap with −6≤k≤6-6 \leq k \leq 6

So we only consider k<3k < 3 combined with −6≤k≤6-6 \leq k \leq 6

This gives us: −6≤k<3-6 \leq k < 3 where kk is an integer.

Therefore: k∈{−6,−5,−4,−3,−2,−1,0,1,2}k \in \{-6, -5, -4, -3, -2, -1, 0, 1, 2\}


The number of possible values of kk is 9.

Note that k=3k = 3 would make the second equation have exactly one repeated root (since D2=0D_2 = 0), which violates our "two distinct roots" condition.

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