Quadratic Equations | Exercise 4.3

Question 2

Find the values of kk for each of the following quadratic equations, so that they have two equal roots.

(i) 2x2+kx+3=02x^2 + kx + 3 = 0

(ii) kx(x2)+6=0kx (x - 2) + 6 = 0

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Solution
Understand the Question
  • A standard quadratic equation is given by ax2+bx+c=0ax^2 + bx + c = 0 (where a0a \ne 0).
  • The nature of the roots is determined by the discriminant D=b24acD = b^2 - 4ac:
    • For two equal real roots, the discriminant must be zero (b24ac=0b^2 - 4ac = 0).
  • We determine coefficients a,b,a, b, and cc for each equation, substitute them into b24ac=0b^2 - 4ac = 0, and solve for kk.
  • Remember: The coefficient of x2x^2 (aa) must not be zero for the equation to remain quadratic.

(i) 2x2+kx+3=02x^2 + kx + 3 = 0

Step 1 · Apply Discriminant Condition and Solve for kk

Comparing 2x2+kx+3=02x^2 + kx + 3 = 0 with ax2+bx+c=0ax^2 + bx + c = 0: a=2,b=k,c=3a = 2, \quad b = k, \quad c = 3

For two equal roots, the discriminant must be zero (b24ac=0b^2 - 4ac = 0):

k24(2)(3)=0k224=0k2=24k=±24=±4×6=±26\begin{aligned} k^2 - 4(2)(3) &= 0 \\ k^2 - 24 &= 0 \\ k^2 &= 24 \\ k &= \pm\sqrt{24} \\ &= \pm\sqrt{4 \times 6} \\ &= \pm 2\sqrt{6} \end{aligned}
Answer

(i) k=±26k = \pm 2\sqrt{6}

(ii) kx(x2)+6=0kx (x - 2) + 6 = 0

Step 1 · Expand and Apply Discriminant Condition

Expanding the equation: kx22kx+6=0kx^2 - 2kx + 6 = 0

Comparing with ax2+bx+c=0ax^2 + bx + c = 0: a=k,b=2k,c=6a = k, \quad b = -2k, \quad c = 6

For two equal roots, b24ac=0b^2 - 4ac = 0:

(2k)24(k)(6)=04k224k=04k(k6)=0\begin{aligned} (-2k)^2 - 4(k)(6) &= 0 \\ 4k^2 - 24k &= 0 \\ 4k(k - 6) &= 0 \end{aligned}

Step 2 · Solve for kk and Check Validity

From 4k(k6)=04k(k - 6) = 0: 4k=0ork6=0    k=0ork=64k = 0 \quad \text{or} \quad k - 6 = 0 \implies k = 0 \quad \text{or} \quad k = 6

If k=0k = 0, the original equation simplifies to 6=06 = 0, which is not a quadratic equation (a0a \ne 0).

Therefore, k0k \ne 0, leaving: k=6k = 6

Answer

(ii) k=6k = 6

Common Mistakes
  • Missing Negative Root in (i): When solving k2=24k^2 = 24, forgetting the ±\pm sign gives only +26+2\sqrt{6} instead of both +26+2\sqrt{6} and 26-2\sqrt{6}.
  • Retaining k=0k = 0 in (ii): Failing to reject k=0k = 0. For ax2+bx+c=0ax^2 + bx + c = 0 to be a valid quadratic equation, the coefficient of x2x^2 cannot be zero (a=k0a = k \ne 0).
  • Expansion Error in (ii): Forgetting to distribute kxkx to both terms inside the parentheses before identifying a,b,a, b, and cc.

More questions in Exercise 4.3

Q1

Find the nature of the roots of the following quadratic equations. If the real roots exist, find them:

(i) 2x23x+5=02x^2 - 3x + 5 = 0

(ii) 3x243x+4=03x^2 - 4\sqrt{3}x + 4 = 0

(iii) 2x26x+3=02x^2 - 6x + 3 = 0

Q2

Find the values of kk for each of the following quadratic equations, so that they have two equal roots.

(i) 2x2+kx+3=02x^2 + kx + 3 = 0

(ii) kx(x2)+6=0kx (x - 2) + 6 = 0

Q3

Is it possible to design a rectangular mango grove whose length is twice its breadth, and the area is 800 m2800 \text{ m}^2? If so, find its length and breadth.

Q4

Is the following situation possible? If so, determine their present ages.

The sum of the ages of two friends is 2020 years. Four years ago, the product of their ages in years was 4848.

Q5

Is it possible to design a rectangular park of perimeter 80 m80\text{ m} and area 400 m2400\text{ m}^2? If so, find its length and breadth.

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