Question

解方程21%=42 45%x-20%x=200 x-40%=360 5分之x+x=360

Answer 1

x+1/x=5

(x+1/x)^2=25

x^2+(1/x)^2+2=25

x^2+(1/x)^2=23

(x-1/x)^2=x^2+(1/x)^2-2

=23-2

=21