设u=√(a-1)>=0,v=|b|>=0,由3√(a-1)+5|b|=7得u=(7-v)/3>=0,∴0<=v<=7.
s=2u-3v=(14-11v)/3,
w=15s+2=72-55v,是v的减函数,
v=0时s取最大值72.