Question

Find the sum of $21$ terms of A.P $-30,-27,-24,...$

Answer 1

$a=-30,d=-27-(-30)=-27+30=3$

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

$S_{21}=\frac{21}{2}[2\times -30+(21-1)\times 3]$

$S_{21}=\frac{21}{2}[-60+\left(20\right)\times 3]$

$S_{21}=\frac{21}{2}[-60+60]$

$S_{21}=\frac{21}{2}\times 0=0$

$\therefore {21}^{th}$ term of the A.P is $0$