00:01
In the expansion of 1 plus x the whole power n, it's given that 5th, 6th, 7th terms, that is t5, t6, t7 are in ap.
00:15
So, what are the coefficients of 5th, 6, 7th terms are what? nc4, nc5, nc6.
00:25
So it's given that these are in ap.
00:27
So what is the formula for ap? 2 into nc5 is equal to nc4 plus.
00:34
Nc6 always remember there is a very nice formula ncr divided by ncr minus 1 is equal to n minus r plus 1 divided by r so this is a beautiful formula so let's use this so what we do now is let's divide this equation both sides by nc5 so i'll be getting 2 is equal to nc 4 divided by nc 5 plus nc6 divided by nc5 i can write this as 1 by nc5 divided by nc 4 plus nc6 divided by nc5.
01:16
So 2 is equal to 1 by now let's apply this formula.
01:20
So for this part r is 5...