Question

Someone please help !!

What is the coefficient of x^6y^4 in the expansion of (x+y)^10
A.1
B.5
C.210
D.it does not exist

Answer 1

Looking at Pascal's Triangle, specifically at the row that starts with 1, 10, etc we see the value 210 in the 4th slot (start the count at 0) since 10-6 = 4

Or you can use the combination formula nCr to get the same result

nCr = (n!)/(r!*(n-r)!)
10C4 = (10!)/(4!(10-4)!)
10C4 = (10!)/(4!*6!)
10C4 = (10*9*8*7*6!)/(4!*6!)
10C4 = (10*9*8*7)/(4!)
10C4 = (10*9*8*7)/(4*3*2*1)
10C4 = 210

Either way, the final answer is Choice C) 210