(a^20 *a^20) /(a^17 *a^19)=a^40 /a^(17+19)=a^40 /a^36=a^(40-36)=a^4;
a=5; 5^4=5*5*5*5=625;
a=-3/11; (-3/11)^4=(3*3*3*3 )/ 11*11*11*11=81/(121*121)=81/14641
a=2,8=28/10=14/5; (14/5)^4=(14^4) /(5^4)=196*196) / 625=38416/625=
=61,448
a=-40; (-40)^4=40^4=40*40*40*40=2560000