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KC Sinha Mathematics Solution Class 11 Chapter 11 द्विघात समीकरण Quadratic Equation) Exercise 11.1

Exercise 11.1

Solve the following equations by factorization method

Question 1

2 x^{2}+3=0
Sol :
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=0
Sol :
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=0
Sol :
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=0
Sol :
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=0



Question 6

9 x^{2}-12 x+20=0

Sol :
(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=0
Sol :
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=0
Sol :
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}=0
Sol :
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}=0
Sol :
\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=0
Sol :
\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}}=0
Sol :
\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}=0
Sol :
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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