10th Class Mathematics Solved Paper - Mathematics-2018

  • question_answer 9) Find the sum of first 8 multiples of 3.

    Answer:

    First 8 multiples of 3 are 3, 6, 9............. up to 8 terms
    We can observe that the above series is an AP with
    \[a=3,\text{ }d=6-3=3,n=8\]
    Sum of n terms of an A.P. is given by,
                              \[{{S}_{n}}=\frac{n}{2}[2a+(n-1)d]\]
                \[\therefore \]      \[{{S}_{8}}=\frac{8}{2}[2\times 3+(8-1)(3)]\]
                                 \[=4[6+7\times 3]\]
                                 \[=4[6+21]\]
                                 \[=4\times 27\]
                \[\Rightarrow \]   \[{{S}_{8}}=108\]


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