a^2+b^2=1a-b=1/2，(a-b)^2=1/4(a-b)^2=a^2-2ab+b^2=a^2+b^2-2ab=1-2ab=1/42ab=3/4, ab=3/8(a+b)^2=a^2+b^2+2ab=1+3/4=7/4故：a^2b^2=(ab)^2=9/64(a+b)^4=[(a+b)^2]^2=(7/4)^2=49/16
