Question

Calculați: 3+6+9+12+...+102.​

Answer 1

Step-by-step explanation:

a=3,d=3

an=a+(n-1)d

102=3+(n-1)3

99=3n-3

96=3n

n=32

S32=32/2[3+120]

=16(123)

=1968

Answer 2

Answer:

1785

Step-by-step explanation:

We have to calculate the given sum: 3+6+9+12+........+102

Clearly this is the sum of first (102/3)=34 multiples of 3.

Let, S= 3+6+9+12+.............+102

[Taking 3 as common from each number of the sum]

=3(1+2+3+4+.........+34)

=3[34*(34+1)/2]

{Since sum of n natural numbers starting from 1 is given by n(n+1)/2}

=3*17*35

=1785

Therefore, the value of the given sum will be 1785. (Answer)