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Question Number 74142 by MASANJAJ last updated on 19/Nov/19

Answered by Rio Michael last updated on 19/Nov/19

$$T_7=a+6d=6---\left(1\right)$$$$T_{18}=a+17d=22---\left(2\right)$$$$eqn\left(2\right)-eqn\left(1\right)\Rightarrow 11d=16$$$$d=\frac{16}{11}$$$$a+6d=6$$$$\Rightarrow a+6\left(\frac{16}{11}\right)=6$$$$a=6(1-\frac{16}{11})=\frac{-30}{11}$$$$S_n=\frac{1}{2}n[2a+(n-1)d]=\frac{1}{2}n[-\frac{60}{11}+(n-1)\frac{16}{11}]=252$$$$512=-\frac{60}{11}n+\frac{16}{11}n-\frac{16}{11}$$$$5648=-44n$$$$n\notin {\mathbb{Z}}^{+}$$$$$$

Commented by MASANJAJ last updated on 19/Nov/19

Thanks sir

Commented by MASANJAJ last updated on 19/Nov/19

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