JEE Main 2021 · 18 March · Morning Shift · Mathematics · Quadratic Equation
A. 1.5+31 . 5 + \sqrt{3}1.5+3
Let x=3+14+13+14+13+…∞x = 3 + \dfrac{1}{4 + \dfrac{1}{3 + \dfrac{1}{4 + \dfrac{1}{3 + \ldots \infty}}}}x=3+4+3+4+3+…∞1111
So, x=3+14+1x=3+14x+1xx = 3 + \dfrac{1}{4 + \dfrac{1}{x}} = 3 + \dfrac{1}{\dfrac{4x + 1}{x}}x=3+4+x11=3+x4x+11
⇒(x−3)=x(4x+1)⇒ \left(x - 3\right) = \dfrac{x}{( 4x + 1 )}⇒(x−3)=(4x+1)x
⇒(4x+1)(x−3)=x⇒ ( 4x + 1 ) ( x - 3 ) = x⇒(4x+1)(x−3)=x
⇒4x2−12x+x−3=x⇒ 4{x}^{2} - 12x + x - 3 = x⇒4x2−12x+x−3=x
⇒4x2−12x−3=0⇒ 4{x}^{2} - 12x - 3 = 0⇒4x2−12x−3=0
x=12±(12)2+12×42×4=12±12(16)8x = \dfrac{12 \pm \sqrt{( 12{)}^{2} + 12 \times 4}}{2 \times 4} = \dfrac{12 \pm \sqrt{12 ( 16 )}}{8}x=2×412±(12)2+12×4=812±12(16)
=12±4×238=3±232= \dfrac{12 \pm 4 \times 2\sqrt{3}}{8} = \dfrac{3 \pm 2\sqrt{3}}{2}=812±4×23=23±23
x=32±3=1.5±3.x = \dfrac{3}{2} \pm \sqrt{3} = 1 . 5 \pm \sqrt{3} .x=23±3=1.5±3.
But only positive value is accepted
So, x=1.5+3x = 1 . 5 + \sqrt{3}x=1.5+3