13−√8×3+√83+√8=3+√89−8
=3+√8
1√8−√7×√8+√7√8+√7=√8+√78−7
=√8+√7
Similarly, by rationalising we get
1√7−√6=√7+√6
1√6−√5=√6+√5
and 1√5−2=√5+2
So, 13−√8−1√8−√7+1√7−√6−1√6−√5+1√5−2
=3+√8−(√8+√7)+(√7+√6)−(√6+√5)+(√5+2)
=3+√8−√8−√7+√7+√6−√6−√5+√5+2
= 5