1.Relation and Function
medium

माना कि एक फलन $f: R \rightarrow R$ सभी $x , y \in R$ के लिए $f( x + y )=f( x ) f( y )$ को संतुष्ट करता है तथा $f(1)=3$ है। यदि $\sum_{i=1}^{ n } f( i )=363$, तो $n$ बराबर है

A

$6$

B

$5$

C

$7$

D

$4$

(JEE MAIN-2020)

Solution

$f(x+y)=f(x) f(y)$

put $x = y =1 \quad f(2)=(f(1))^{2}=3^{2}$

put $x=2, y=1 \quad f(3)=(f(1))^{3}=3^{3}$ :

Similarly $f(x)=3^{x}$

$\sum_{i=1}^{n} f(i)=363 \Rightarrow \sum_{i=1}^{n} 3^{i}=363$

$\left(3+3^{2}+\ldots+3^{n}\right)=363$

$\frac{3\left(3^{n}-1\right)}{2}=363$

$3^{n}-1=242 \Rightarrow 3^{n}=243$

$\Rightarrow n =5$

Standard 12
Mathematics

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