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Solve the equation $2x^2 - 7x - 3 = 0$ - Leaving Cert Mathematics - Question 3 - 2018

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Solve the equation $2x^2 - 7x - 3 = 0$. Give each answer correct to 2 decimal places. Solve the simultaneous equations below to find the value of $a$ and the value ... show full transcript

Worked Solution & Example Answer:Solve the equation $2x^2 - 7x - 3 = 0$ - Leaving Cert Mathematics - Question 3 - 2018

Step 1

Solve the equation $2x^2 - 7x - 3 = 0$

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Answer

To solve this quadratic equation, we will use the quadratic formula:

x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

Here, a=2a = 2, b=7b = -7, and c=3c = -3. Plugging in these values gives:

x=(7)±(7)242(3)22x = \frac{-(-7) \pm \sqrt{(-7)^2 - 4 \cdot 2 \cdot (-3)}}{2 \cdot 2}

Calculating inside the square root:

x=7±49+244=7±734x = \frac{7 \pm \sqrt{49 + 24}}{4} = \frac{7 \pm \sqrt{73}}{4}

Now compute the two possible values for xx:

  1. x1=7+7343.89x_1 = \frac{7 + \sqrt{73}}{4} \approx 3.89
  2. x2=77340.39x_2 = \frac{7 - \sqrt{73}}{4} \approx -0.39

Therefore, the solutions to the equation are approximately x3.89x \approx 3.89 and x0.39x \approx -0.39.

Step 2

Solve the simultaneous equations below to find the value of $a$ and the value of $b$

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Answer

We have the two equations:

  1. 2a+3b=152a + 3b = 15 (Equation 1)
  2. 5a+b=85a + b = -8 (Equation 2)

To solve this system, we can express bb from Equation 2:

b=85ab = -8 - 5a

Now substitute this expression for bb into Equation 1:

2a+3(85a)=152a + 3(-8 - 5a) = 15

Simplifying this gives:

2a2415a=152a - 24 - 15a = 15

Combine the terms:

13a24=15-13a - 24 = 15

Adding 24 to both sides:

13a=39-13a = 39

Dividing by -13:

a=3a = -3

Now substitute a=3a = -3 back into the expression for bb:

b=85(3)=8+15=7b = -8 - 5(-3) = -8 + 15 = 7

Thus, the values are a=3a = -3 and b=7b = 7.

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