If S29=99 and S99=29 find S128

1.	If S29=99 and S99=29 find S128
Answer» Tnx but S means sum..... Solve it by taking sum of 99 terms..... ? S29= a + 28d.S99= a + 98d... .a+28d=99.a+98d=29.==> -70d= - 70===> d=1.... a+28d=99==>a= 99-28==> a= 71...S128=a+127d==) 71 + 127===> 198.......?

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