A11=23;a21=43; s10=?

Question

A11=23;a21=43; s10=?

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Answer 1

10 d = a21 - a11= 43 - 23=20; 10 d = 20;
 d = 2; 
a11= a1 + 9d;
a1 + 9*2 = 23;
a1 = 5; 
S10 = (2a1+ d*9)*10/2=(2*5+ 2*9) *5 = 28*5 = 140