Question

Find the sum of first n terms of AP and the sum of first 20 terms : 5+11+17+23....​

Answer 1

Finding common difference of given A.P. -

$\text{d} = 11 - 5 = 6$

Formula for sum of first n terms of an A.P. -

$\fbox{ \frac{n}{2}\big[2a+(n-1)d\big] }$

Sum of first 20 terms -

$\frac{20}{2}\big[2(5) + (20-1)6\big]\\\longrightarrow 10[10 + 19\times6] = 10[124] = 1240$