（ab）的2m次方等于（7*4）^2=784
已知：a^m=7,b^m=4,则：(ab)^2m=[(ab)^m]^2=[(a^m)*(b^m)]^2=(7*4)^2=784
（ab）^2m=(ab)^m*(ab)^m=a^m*b^m*a^m*b^m=7*4*7*4
a^m=7,b^m=4,(ab)^2m=[(ab)^m]^2=[(a^m)*(b^m)]^2=(7*4)^2=784
