Find t sub n if t sub 3 is equal to 15 and t sub 8 is equal to 50
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document.write( "If the sequence is arithmetic, we use the formula:\r\n" );
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document.write( "three times. Once for n=3. once for n=8, and once using\r\n" );
document.write( "the values for t1 and d.\r\n" );
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document.write( "Substitute n = 3\r\n" );
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document.write( "Substitute 15 for t3\r\n" );
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document.write( "Substitute n = 8\r\n" );
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document.write( "Substitute 50 for t8\r\n" );
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document.write( "So you have this system of equations to solve:\r\n" );
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document.write( "Solve that system by substitution or elimination and\r\n" );
document.write( "get\r\n" );
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and 
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document.write( "Therefore \r\n" );
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document.write( "becomes\r\n" );
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document.write( "Edwin
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