If 18C5 + 18C6 =nС13, then n =
a) 19
b) 20
c) 18
d) 24
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Answers
Answered by
1
Answer:
20 is correct
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Answered by
2
Answer:
20
Step-by-step explanation:
18
C
15
+2(
18
C
16
)+
17
C
16
+1=
n
C
3
18
C
15
+
18
C
16
+
18
C
16
+
17
C
16
+1=
n
C
3
[∵
n
C
r+1
+
n
C
r
=
n+1
C
r
]
19
C
16
+
18
C
16
+
17
C
16
+
17
C
17
=
n
C
3
19
C
16
+
18
C
16
+
18
C
17
=
n
C
3
19
C
16
+
19
C
17
=
n
C
3
20
C
17
=
n
C
3
20
C
3
=
n
C
3
⇒n=20.
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