Question #2db64

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Answer 1 · 2018-01-19T23:31:24

$3/2$

$sqrt(4x^2+8x)-sqrt(x^2+1)-sqrt(x^2+x) =$

$=sqrt(x^2+2x)-sqrt(x^2+1)+sqrt(x^2+2x)-sqrt(x^2+x) =$

$= ((x^2+2x)-(x^2+1))/(sqrt(x^2+2x)+sqrt(x^2+1))+((x^2+2x)-(x^2+x))/(sqrt(x^2+2x)+sqrt(x^2+x)) =$

$=(2x-1)/(sqrt(x^2+2x)+sqrt(x^2+1))+x/(sqrt(x^2+2x)+sqrt(x^2+x)) =$

$(2-1/x)/(sqrt(1+2/x)+sqrt(1+1/x^2))+1/(sqrt(1+2/x)+sqrt(1+1/x))$

and then

$lim_(x->oo)sqrt(4x^2+8x)-sqrt(x^2+1)-sqrt(x^2+x) =2/2+1/2 = 3/2$

Answer 2 · 2018-01-20T11:53:02

Answer to the question: "What is the limit as x approaches infinity of $sqrt(4x^2+8x)-sqrt(x^2+1)-sqrt(x^2+x)?$

$3/2$

$sqrt(4x^2+8x)-sqrt(x^2+1)-sqrt(x^2+x) =$

$x(2sqrt(1+2/x)-sqrt(1+1/x^2)-sqrt(1+1/x))$

now calling $y = 1/x$ we have

$lim_(x->oo)sqrt(4x^2+8x)-sqrt(x^2+1)-sqrt(x^2+x)equiv lim_(y->0)1/y(2 sqrt(1+2y)-sqrt(1+y^2)-sqrt(1+y))$

now developing $2 sqrt(1+2y)-sqrt(1+y^2)-sqrt(1+y)$ in series aroung $y=0$ we get

$2 sqrt(1+2y)-sqrt(1+y^2)-sqrt(1+y) approx (3 y)/2 - (11 y^2)/8 + (15 y^3)/16 - (139 y^4)/128+O(y^5)$

and then

$lim_(y->0)1/y(2 sqrt(1+2y)-sqrt(1+y^2)-sqrt(1+y))=$

$lim_(y->0) (3 )/2 - (11 y)/8 + (15 y^2)/16 - (139 y^3)/128+O(y^3) = 3/2$