show that

ab+ac+bc=<a^2+b^2+c^2

i've done a similar example and trying to do what i did there with this one but just not working out. below is the one i've already done

2ab=<a^2+b^2

i did like this

since

(a-b)^2=a^-2ab+b^2

thus

a^2+b^2=(a-b)^2+2ab

but

(a-b)^2>=0

then

a^2+b^2>=2ab