Explanation:
1/(1 + sqrt(2))
remember the fact that
(a + b)(a - b) = a² - b²
so, I can multiply the fraction by
(1 - sqrt(2))/(1 - sqrt(2))
and I get
(1 - sqrt(2))/(1 - 2) = -(1 - sqrt(2)) = sqrt(2) - 1
the same for
1/(sqrt(2) + sqrt(3))
we multiply by
(sqrt(2) - sqrt(3))/(sqrt(2) - sqrt(3))
and get
(sqrt(2) - sqrt(3))/(2 - 3) =
= -(sqrt(2) - sqrt(3)) = sqrt(3) - sqrt(2)
and we do this for every fraction.
we get
sqrt(2) - 1 + sqrt(3) - sqrt(2) + sqrt(4) - sqrt(3) + sqrt(5) - sqrt(4) + sqrt(6) - sqrt(5) + sqrt(7) - sqrt(6) + sqrt(8) - sqrt(7) + sqrt(9) - sqrt(8) =
= sqrt(9) - 1 =
= 3 - 1 = 2