Question

find the sum of the 21 terms of an AP 5,8,11,14...​

Answer 1

Answer:

Hope this answer helps you

Answer 2

Sum ÷

Find the sum of 21st term of AP 5,8,11,14........

Solution-

$a = > 5 \\ \\ d = > a2 - a1 \\ \\ d = > 8 - 5 \\ \\ d = > 3$

$\color{red} \boxed{sn = > \frac{n}{2} (2a + (n - 1) \times d)}$

$n = > 21 \\ \\ s21 = > \frac{21}{2} (2 \times 5 + (21 - 1) \times 3) \\ \\ s21 = > \frac{21}{2} (10 + 20 \times 3) \\ \\ s21 = > \frac{21}{2} (10 + 60) \\ \\ s21 = > \frac{21}{2} (70) \\ \\ s21 = > 21 \times 35 \\ \\ \boxed{s21 = > 735}$

The sum of 21st term of AP 5,8,11.......is 735

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One other important Formula

$\boxed{an = > a + (n - 1) \times d}$

Hope it's helps you