Chứng minh rằng:
a, \(2\left(a^4+b^4\right)\ge\left(a+b\right)\left(a^3+b^3\right)\)
b, \(3\left(a^4+b^4+c^4\right)\ge\left(a+b+c\right)\left(a^3+b^3+c^3\right)\)
c, \(\dfrac{a^2}{b}+\dfrac{b^2}{c}+\dfrac{c^2}{a}\ge a+b+c\)
d, \(a^2+b^2+c^2+d^2\ge ab+ac+ad\)