3a+2b=1
b=(1-3a)/2
9a方+4b方
=9a²+4[(1-3a)/2]²
=9a²+(1-3a)²
=9a²+1-6a+9a²
=18a²-6a+1
=2(9a²-3a+1/4)+1/2
=2[(3a-1/2)²+1/2
当a=1/6时,9a²+4b²有最小值1/2.