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Mathematics

Solve the following equation for x and give your answer correct to one decimal place :

x2 - 8x + 5 = 0

Quadratic Equations

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Answer

Comparing x2 - 8x + 5 = 0 with ax2 + bx + c = 0 we get,

a = 1, b = -8 and c = 5.

We know that,

x = b±b24ac2a\dfrac{-b \pm \sqrt{b^2 - 4ac}}{2a}

Substituting values of a, b and c in above equation we get,

x=(8)±(8)24.(1).(5)2(1)=8±64202=8±442=8±2112=4±11=4+11 and 411=4+3.3 and 43.3=7.3 and 0.7\Rightarrow x = \dfrac{-(-8) \pm \sqrt{(-8)^2 - 4.(1).(5)}}{2(1)} \\[1em] = \dfrac{8 \pm \sqrt{64 - 20}}{2} \\[1em] = \dfrac{8 \pm \sqrt{44}}{2} \\[1em] = \dfrac{8 \pm 2\sqrt{11}}{2} \\[1em] = 4 \pm \sqrt{11} \\[1em] = 4 + \sqrt{11} \text{ and } 4 - \sqrt{11} \\[1em] = 4 + 3.3 \text{ and } 4 - 3.3 \\[1em] = 7.3 \text{ and } 0.7

Hence, x = 7.3 and 0.7

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