Question

5^2+5^3+.......+5^100?

Answer 1

s=5^2+5^3+5^4+.............+5^100

5s=5^3+5^4+.................+5^101

5s-s=(5^3+5^4+......+5^101)-(5^2+5^3+........+5^100)

=>4s=5^2-5^101

Answer 2

A=5^2+5^3+.......+5^100

5A=5^3+.......+5^100+5^101

suy ra 4A=5^101-5^2

suy ra A=(5^101-5^2)/4