Question

Calculate the sum of the following series:
(a) -20 – 13 – 6+. ... to 15 terms.
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Answer 1

Answer:

The answer is 435

Step-by-step explanation:

Given series is

$-20-13-6+...\text{15 terms}$

Clearly its and A.P. with first term $a=-20$ and common difference $d=7$

Thus using the formula

              $\boxed{\text{S}_n=\dfrac{n}{2}(2a+(n-1)d)}$

Required sum

                     $=\dfrac{15}{2}[2(-20)+(15-1)(7)]\\\\=\dfrac{15}{2}(-40+98)\\\\=15(-20+49)=15(29)=435$