Question

the 10th term of an arithmetic sequence is 20 and its 20th term is 10 . what is its common difference
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Answer 1

$a_1_0 =20\\\\a+9d=20$________{i}

$a_2_0 = 10\\a+19d =10$_____{ii}

from {i} and {ii}

   a  +   19d   =     10

(-)a  +  (-)9d   =   (-)20

             $10d=(-10)$

                $d = (-1)$

hence    $d = (-1)$

a = first term

d = common difference