The sides of a triangle are x^(2 )+x+1, 2x+1,x^2-1, prove that the 
largest angle is 120 degrees, and find range of x.

The biggest side is x^(2) + x + 1

so findout the angle opposite to that by

Cos(A) = (b^2 + c^2-a^2)/2*b*c

which gives Cos(A) = -1/2 ie 120degress