R1+R2=10R并=R1R2/（R1+R2）=R1R2/10 用数学的君子不等式：R1R2 =1/4*（2√R1R2）^2 ≤1/4 *(R1+R2)^2 =1/4 * 100 =25 R并 ≤25/10=2.5最大电阻:2.5欧
