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Irina-Kira [14]
3 years ago
13

The seventh term of an arithmetic sequence is 21, and the tenth term is 126. The first term is _____..

Mathematics
2 answers:
garri49 [273]3 years ago
7 0
Pretend that the first term is 21, and the fourth term is 126 
<span>it's the same thing. </span>
<span>126 =21 +3d </span>
<span>105=3d </span>
<span>d=35 </span>
<span>21 =a1 +6(35) </span>
<span>21=a1 +210 </span>
<span>a1=-189</span>
Ivan3 years ago
7 0
a_7=21;\ a_{10}=126\\\\a_{10}-a_7=3d\\\\subtitute\\\\3d=126-21\\3d=105\ \ \ |divide\ both\ sides\ by\ 3\\d=35\\\\a_1=a_7-6d\\\\a_1=21-6\cdot35=21-210=-189\\\\Answer:\boxed{a_1=-189}
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