Question 974074: If 15c3r = 15cr+3, then r =
1. 2
2. 3
3. 4
4. 5
Answer by farohw(175)   (Show Source):
You can put this solution on YOUR website!
Hello,
I remember this from Discrete Math.
15 is a superscript of C and 3r, r+3 are subscripts of C. If 15C3r = 15Cr+3, then we follow the formula:
nCx = nCy where x = y OR x + y = n.
Let n represent 15, x = 3r and y = r +3:
3r = r + 3 and 3r + r + 3 = 15
3r = r + 3 ---> 3r - r =3 then 2r = 3 which is r = 3/2
3r + r + 3 = 15 ---> 4r = 12 which is r = 3
r cannot be a fraction since this will give us 1.5 therefore the answer will be r = 3.
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