Question

find the sum of twenty terms of arthemetic series 2+7+12+................using suitable formula​

Answer 1

Answer:

$sum = \frac{n}{2} (2a + (n - 1)d) \\ \ = \frac{20}{2} (2 \times 2 + 19 \times 5) \\ = 10 \times (4 + 95) \\ = 10 \times 99 \\ = 990$

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Answer 2

$\large{ \underline{ \dag{ \orange{ \bold{ANSWER}}}}}$

$\bold\pink{AP}$ = 2,7,12,.......20

so,

$\bold\blue{Difference (d)}$

= 7-2

= 5

$\bold\blue{a}$ = 2

$\bold\blue{an}$ = 20

Now by using the formula.............

= $sn = \frac{n}{2} ({2a +(n - 1})d)$

= $\frac{20}{2} (2 \times 2 + (20 - 1)5)$

= $10(2 \times 2 + 19 \times 5)$

= $10(4 + 95)$

= $10 \times 99$

= $\bold\green{990}$ is the right answer.

$\huge\red{\ddot\smile}$ hope it helps you ✅✔️