Question

the sum of the first ten terms of a linear sequence is -60 and the sum of the first fifteen terms is -165. Find the 18th term of the sequence​

Answer 1

Answer:

         a₁₈ = - 31

Step-by-step explanation:

$S_{10}=\dfrac{2a_1+d(10-1)}{2}\cdot 10\\\\\dfrac{2a_1+9d}{2}\cdot 10=-60\\\\\dfrac{2a_1+9d}{2}=-6\\\\2a_1+9d=-12\\\\2a_1=-12-9d\\\\\\S_{15}=\dfrac{2a_1+d(15-1)}{2}\cdot 15\\\\\dfrac{2a_1+14d}{2}\cdot 15=-165\\\\\dfrac{2a_1+14d}{2}=-11\\\\2a_1+14d=-22\\\\-12-9d+14d=-22\\\\ 5d=-10 \\\\ d=-2\\\\\\2a_1=-12-9(-2)=-12+18=6\\\\a_1=3\\\\\\ a_{18}=a_1+17d=3+17(-2)=3-34=-31$