Dear Student,
BY BINOMIAL EXPANSION:
(x+a)n=nC0xna0+nC1xn-1a1+nC2xn-2a2+..........+nCnx0an
No. of terms in (x+y+z)^n
⇒[x+(y+z)]^n
⇒[x+(y+z)]^n=nC0xn (y+z)0+nC1xn-1 (y+z)1+nC2xn-2(y+z)2+.....nCnx0(y+z)n
=>No. of terms in (x+y+z)n
⇒(n+1)/2*[(n+2)/2]
⇒(n+1)/2(n+2)/2
Regards
Manshi Arora