1.

If f(x) is a function satisfying f(x+y)=f(x)f(y) for all x,y `in` N such that f(1)=3 and `sum_(x=1)^(n) f(x)=120`. Then, the value of n isA. 4B. 5C. 6D. none of these

Answer» Correct Answer - A
We have,
`f(x+y)=f(x)f(y)" for all "x,yinN`.
`:." "f(x)={f(1)}^(x)=3^(x)" "[becausef(1)=3]`
Now,
`underset(x=1)overset(n)sumf(x)=120`
`rArr" "underset(x=1)overset(n)sum3^(x)=120`
`rArr" "(3(3^(n)-1))/((3-1))=120rArr3^(n)-1=80rArr3^(n)=3^(4)rArrn=4`


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