1/2·1/4·1/8·1/16·1/32·1/64
=(1/2)*(1/2)^2*(1/2)^3*(1/2)^4*(1/2)^5*(1/2)^6
=(1/2)^(1+2+3+4+5+6)
=(1/2)^21=(1/2)^x
x=21