Answer:
steps below
Step-by-step explanation:
5¹+5² = 30 30/<u>6</u> = 5¹
5³+5⁴ = 125 + 625 = 750 750/<u>6</u> = 125 = 5³
5⁵+5⁶ = 18750 18750/6 = 3125 = 5⁵
...........
(5¹⁹⁹¹+5¹⁹⁹²)/<u>6</u> = 5¹⁹⁹¹
sum of every couple consecutive multiple of 5 are multiple of 6, the sum
S= <u>5+ 5^2</u> + 5^3 + ...+ <u>5^1991 + 5 ^1992</u> IS multiple of 6