已知函数f【x】＝3x²－5x＋2　
f【a＋3】＝3(a+3)²-5(a+3)+2=3a²+18a+27-5a-15+2=3a²+13a14
f【a】＋f【3】=3a²-5a+2+27-15+2=3a²-5a+16