document.write( "Question 1184573: A manufacturer knows that their items have a normally distributed lifespan,
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Algebra.Com's Answer #815252 by Edwin McCravy(20060)\"\" \"About 
You can put this solution on YOUR website!
A manufacturer knows that their items have a normally distributed lifespan,
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\n" ); document.write( "life will be longer than 3 years? (Give answer to 4 decimal places.)
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document.write( "The rule is:\r\n" );
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document.write( "You must use the t-distribution when working problems when the population\r\n" );
document.write( "standard deviation (σ) is not known and the sample size is small (n<30).\r\n" );
document.write( "General Correct Rule: If σ is not known, then using t-distribution is\r\n" );
document.write( "correct. If σ is known, then using the normal distribution is correct.\r\n" );
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document.write( "In this problem, σ is known, so we use the normal distribution.  However,\r\n" );
document.write( "since the sample size is 23 we must divide the standard deviation by √23.\r\n" );
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document.write( "On your TI-84, press ON CLEAR 2ND VARS 3\r\n" );
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document.write( "Make the screen read like this:\r\n" );
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document.write( "     normalcdf\r\n" );
document.write( "lower:3\r\n" );
document.write( "upper:99999999\r\n" );
document.write( "μ:3.3\r\n" );
document.write( "σ:1.1/√(23)      <-- the √ key is 2ND x2\r\n" );
document.write( "Paste\r\n" );
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document.write( "Use the down arrow key to highlight Paste\r\n" );
document.write( "Press ENTER\r\n" );
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document.write( "Read this:\r\n" );
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document.write( "normalcdf(3,99999999,3,3,1.1/√(23))\r\n" );
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document.write( "Press ENTER\r\n" );
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document.write( "Read 0.9045554884\r\n" );
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document.write( "Round to 0.9046\r\n" );
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document.write( "Edwin
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