5a²+b ²=6b=√(6-5a²)令y=3a+b=3a+√(6-5a²)y-3a=√(6-5a²)y²-6ay+9a²=6-5a²14a²-6ay+y²-6=0应该有a.b.是实数36y² -4*14*(y²-6)≥05y²-84≤02/5*...