first 3 terms of arithmetic sequence
where the 21st term is -38 and the 50th term is -125
Let
x=21tx=t21=−38y=50ty=t50=−125z=1tz=t1=?
AP:tz=tx⋅(z−yx−y)+ty⋅(x−zx−y)t1=(−38)⋅(1−5021−50)+(−125)⋅(21−121−50)t1=(−38)⋅(−49−29)+(−125)⋅(20−29)t1=−38⋅(4929)+125⋅(2029)t1=−38⋅49+125⋅2029t1=63829t1=22
Let
x=21tx=t21=−38y=50ty=t50=−125z=2tz=t2=?
AP:tz=tx⋅(z−yx−y)+ty⋅(x−zx−y)t2=(−38)⋅(2−5021−50)+(−125)⋅(21−221−50)t2=(−38)⋅(−48−29)+(−125)⋅(19−29)t2=−38⋅(4829)+125⋅(1929)t2=−38⋅48+125⋅1929t2=55129t2=19
d=t2−t1d=19−22d=−3t3=t2+dt3=19−3t3=16
The first 3 terms of arithmetic sequence: 22, 19, 16, ...
