回答：
#4.2^-5 = 3125/4084101 ~~0.0076516#
説明：
#4.2 = 21/5#
#21^5 = (20+1)^5 = 20^5+5*20^4+10*20^3+10*20^2+5*20+1#
#= 3200000 + 800000 + 80000 + 4000 + 100 + 1#
#= 4084101#
#5^5 = 3125#
そう #4.2^-5 = (21/5)^-5 = (5/21)^5 = 5^5/(21^5) = 3125/4084101#
#=1/1306.91232 ~~0.0076516#
