If f:R to R satisfies f(x+y)=f(x)+f(y),...

If `f:R to R ` satisfies f(x+y)=f(x)+f(y), for all x, y `in` R and f(1)=7, then `sum_(r=1)^(n)f( r)` is

A

` (7n(n+1))/(2)`

B

`(7n)/(2)`

C

`(7(n +1))/(2)`

D

`7n(n +1)`

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The correct Answer is:

A

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