Exercise 11.1
Solve the following equations by factorization method
Question 1
2 x^{2}+3=0Sol :
2 x^{2}=-3
x^{2}=-\frac{3}{2}
x=\pm \sqrt{\frac{-3}{2}}=\pm \sqrt{\frac{3}{2}} i
ALTERNATE METHOD
2 x^{2}+3=0
(\sqrt{2} x)^{2}-(\sqrt{3} i)^{2}=0
\left(\sqrt{2} x+\sqrt 3 i\right)(\sqrt{2} x-\sqrt{3} i)=0
\sqrt{2} x+\sqrt3 i=0
\sqrt 2 x=-\sqrt 3 i
x=\frac{-\sqrt{3}}{2}i
and \sqrt 2 x-\sqrt 3i=0
\sqrt2x=\sqrt3i
x=\frac{\sqrt{3}}{2}i
Question 2
x^{2}+x+1=0Sol :
x^{2}+2 \cdot x \cdot \frac{1}{2}+\left(\frac{1}{2}\right)^{2}+1-\left(\frac{1}{2}\right)^{2}=0
\left(x+\frac{1}{2}\right)^{2}+1-\frac{1}{4}=0
\left(x+\frac{1}{2}\right)^{2}+\frac{4-1}{4}=0
\left(x+\frac{1}{2}\right)^{2}+\frac{3}{4}=0
\left(x+\frac{1}{2}\right)^{2}-\left(\frac{\sqrt{3}}{2}i\right)^{2}=0
\left(x+\frac{1}{2}-\frac{\sqrt{3}}{2} i\right)\left(x+\frac{1}{2}+\frac{\sqrt{3}}{2} i\right)=0
x+\frac{1}{2}-\frac{\sqrt3}{2} i=0
x=-\frac{1}{2}+\frac{\sqrt 3}{2}i
and x+\frac{1}{2}+\frac{\sqrt{3}}{2} i=0
x=-\frac{1}{2}-\frac{\sqrt{3}}{2}i
∴ x=-\frac{1}{2} \pm \frac{\sqrt 3}{2} i
Question 3
x^{2}+2 x+5=0Sol :
x^{2}+2 \cdot x \cdot 1+1^{2}+5-1^{2}=0
(x+1)^{2}+5-1=0
(x+1)^{2}+4=0
(x+1)^{2}-(2 i)^{2}=0
(x+1-2i)(x+1+2i)=0
x+1-2i=0
x=-1+2i
and x+1+2i=0
x=-1-2i
∴ x=-1±2i
ALTERNATE METHOD
x^{2}+2 \cdot x \cdot 1+1^{2}+5-1^{2}=0
(x+1)^{2}+5-1=0
(x+1)^{2}+4=0
(x+1)^{2}=-4
x+1=\pm \sqrt{-4}
x+1=±2i
x=-1±2i
Question 4
x^{2}-4 x+7=0Sol :
x^{2}-2 \cdot x \cdot 2+2^{2}+7-2^{2}=0
(x-2)^{2}+3=0
(x-2)^{2}-(\sqrt{3} i)^{2}=0
Question 5
x^{2}-4 x+13=0Question 6
9 x^{2}-12 x+20=0Sol :
(3x)^2-2 \cdot 3 x \cdot 2+2^{2}+20-2^{2}=0
(3 x-2)^{2}+11=0
Question 7
Solve the following equations by using the general expression for roots of a quadratic equation:2 x^{2}-\sqrt{3} x+1=0
Sol :
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2a}i
a=2 ,b=-\sqrt{3} , c=1
=\frac{-(-\sqrt{3}) \pm \sqrt{4 \times 2 \times 1-(-\sqrt{3})^{2}}}{2 \times 2}i
x=\frac{\sqrt{3} \pm \sqrt{8-3}}{4}i
x=\frac{\sqrt{3} \pm \sqrt{5} i}{4}
=\frac{\sqrt{3}}{4} \pm \frac{\sqrt{5}}{4}i
Question 8
17 x^{2}-8 x+1=0Sol :
a=17 , b=-8 , c=1
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2a}i
=\frac{-(-8) \pm \sqrt{4 \times 17 \times 1-(-8)^{2}}}{2 \times 17}i
x=\frac{8 \pm \sqrt{68-64}}{34}i
2=\frac{8 \pm \sqrt{4}}{34}i
x=\frac{8 \pm 2 i}{34}
x=\frac{2(4 \pm i)}{34}
=\frac{4}{17} \pm \frac{1}{17}
Question 12
9 x^{2}+4=0Sol :
a=9 , b=0 , c=4
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2 a}i
=\frac{-0 \pm \sqrt{4 \times 9 \times 4-0^{2}}}{2 \times 9}i
x=\frac{\pm \sqrt{144}}{18}i
x=\pm \frac{12 i}{18}
=\pm \frac{2}{3} i
Question 17
\sqrt{3} x^{2}-\sqrt{2} x+3 \sqrt{3}=0Sol :
a=\sqrt{3}, b=-\sqrt{2} ,c=3 \sqrt{3}
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2 a}i
x=\frac{-(-\sqrt{2}) \pm \sqrt{4 \times \sqrt{3} \times 3 \sqrt{3}-(-\sqrt{2})^{2}}}{2 \sqrt{3}}i
x=\frac{\sqrt{2} \pm \sqrt{36-2}}{2 \sqrt{3}}i
x=\frac{\sqrt{2} \pm \sqrt{34}}{2 \sqrt{3}}i
Question 27
x^{2}-2 x+\frac{3}{2}=0Sol :
\frac{2 x^{2}-4 x+3}{2}=0
2 x^{2}-4 x+3=0
a=2 , b=-4 , c=3
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2 a}i
x=\frac{-(-4) \pm \sqrt{4 \times 2 \times 3-(-4)^{2}}}{2 \times 2}i
x=\frac{4 \pm \sqrt{24-16}}{4}i
x=\frac{4 \pm \sqrt{8} i}{4}
=\frac{4 \pm 2 \sqrt{2} i}{4}
x=\frac{2(2 \pm \sqrt{2} i)}{4}
x=\frac{2 \pm \sqrt{2} i}{2}
x=1 \pm \frac{\sqrt{2}}{2} i
Question 28
x^{2}+\frac{x}{\sqrt{2}}+1=0Sol :
\frac{\sqrt{2} x^{2}+x+\sqrt{2}}{\sqrt{2}}=0
\sqrt 2 x^{2}+x+\sqrt2=0
a=\sqrt{2} , b=1 , c=\sqrt{2}
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2 a}i
x=\frac{-1 \pm \sqrt{4 \times \sqrt{2} \times \sqrt{2}-1^{2}}}{2 \sqrt{2}}
x=\frac{-1 \pm \sqrt{8-1}}{2 \sqrt{2}}i
x=\frac{-1 \pm \sqrt{7} i}{2 \sqrt{2}}
Question 30
x^{2}+x+\frac{1}{\sqrt{2}}=0Sol :
\frac{\sqrt{2} x^{2}+\sqrt{2} x+1}{\sqrt{2}}=0
\sqrt{2} x^{2}+\sqrt{2} x+1=0
a=\sqrt{2}, b=\sqrt{2}, c=1
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2 a}i
x=\frac{-\sqrt{2} \pm \sqrt{4 \sqrt{2} \times 1-(\sqrt{2})^{2}}i}{2 \sqrt{2}}
x=\frac{-\sqrt{2} \pm \sqrt{4 \sqrt{2}-2}}{2 \sqrt{2}}i
x=\frac{\sqrt{2} \pm \sqrt{2(2 \sqrt{2}-1)}}{2 \sqrt{2}}i
x=\sqrt2\left(\frac{-1 \pm \sqrt{2 \sqrt{2}-1}i}{2 \sqrt{2}}\right)
x=-\frac{1 \pm \sqrt{2 \sqrt{2}-1}}{2}
Question 31
\sqrt{5} x^{2}+x+\sqrt{5}=0Sol :
a=\sqrt{5} ,b=1, c=\sqrt{5}
x=\frac{-b \pm \sqrt{4 a c-b^{2}}}{2 a}i
x=\frac{-1 \pm \sqrt{4 \times \sqrt{5} \times \sqrt{5}-1^{2}}i}{2 \times \sqrt{5}}
x=\frac{-1 \pm \sqrt{20-1}}{2 \sqrt{5}}i
x=\frac{-1 \pm \sqrt{19}}{2 \sqrt{5}}i
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