[tex] a_1=153\\a_2=139\\a_3=125\\\vdots\\\\d=a_2-a_1\to d=139-153=-14 [/tex]
The formula of the sum of the arithmetic sequence:
[tex]S_n=\dfrac{2a_1+(n-1)d}{2}\cdot n[/tex]
We have:
[tex]a_1=153\\d=-14\\n=22[/tex]
substitute
[tex] S_{22}=\dfrac{2\cdot153+(22-1)\cdot(-14)}{2}\cdot22=(306+21\cdot(-14))\cdot11=132 [/tex]
