a*a+b*b>=2*a*b;
b*b+c*c>=2*b*c;
c*c+a*a>=2*c*a;
each = is true only if x==y


2(a*a+b*b+c*c)>=2(a*b+b*c+c*d)

the = is true only if
a==b==c

There are two parts in this question.
a) Prove that if a quadratic equation has consective integers as its coefficients, then it has no real roots.
B), Prove or disprove the theorem that if a quadratic equation has three consecutive integers as its coefficients, then it has no real roots.

Well, I did the second part as :
the quadratic equation (n-1)x*x+nx+(n+1)=0, where n>=2.
then b*b-4ac<0,
it is true.

What should I write the first part? How to tell them apart?
Thanks again.


the third question is ' Prove that f(n)=n*n*n-n*n-4 is a composite number. (ie, not a prime number) for all integers n>2.

这可是基本功啊！！