matematika



Prove that the equation has no real solutions

Prove that the equation has no real solutions

Postby Math Tutor » Thu Jun 21, 2012 5:19 pm

Prove that the equation
sinx - x^2 - x - 1 = 0
has no real solutions.


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Re: Prove that the equation has no real solutions

Postby Math Tutor » Sat Jun 30, 2012 1:41 am

Any suggestions?
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Re: Prove that the equation has no real solutions

Postby idontknow » Sun Jul 01, 2012 6:38 am

sinx=x²+x+1 but -1 \le sinx \le 1
so we have the system
lx²+x+1 \ge -1 (1)
lx²+x+1 \le 1 (2)

(1)x²+x+1 \ge -1 => x²+x+2 \ge 0
Let's suppose that it isi equality then it doesn't have solutions.If it is only x²+x+2>0 the D<0 and a>0 so every x is possible
(2)x²+x+1 \le 1 => x²+x \le 0 .Lets suppose it is an equality then the roots are -1,0 if we put them into sinx=x²+x+1 we will see that it has no solutins.Now if x²+x<0 well it more than obvious that it is impossible.With that the system has no real solutions and the whole problem
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Re: Prove that the equation has no real solutions

Postby shemet » Fri Jul 13, 2012 8:07 am

idontknow wrote:(2)x2+x+1 < 1 => x2+x < 0 .Lets suppose it is an equality then the roots are -1,0 if we put them into sinx=x2+x+1 we will see that it has no solutins.Now if x2+x<0 well it more than obvious that it is impossible.With that the system has no real solutions and the whole problem


Bull·shit!!!
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Re: Prove that the equation has no real solutions

Postby Guest » Tue Aug 14, 2012 5:39 pm

Just take a look at the graph of f(x) = x^2+x+1 and g(x) = sin (x) and you're almost done. In the critical area
when x in [-1,0], the values of g(x) = sin (x) are negative. For the rest of the values there is no worry since
max(sin(x)) = 1.

YS
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