document.write( "Question 1039196: use the cosine of a sum and cosine of a difference identities to find cos(s+t) and cos(s-t).\r
\n" ); document.write( "\n" ); document.write( "sin s= -4/5 in Q IV
\n" ); document.write( "sin t= 12/13 in Q II\r
\n" ); document.write( "\n" ); document.write( "what is cos(s+t)?
\n" ); document.write( "what is cos(s-t)?\r
\n" ); document.write( "\n" ); document.write( "thank you in advance!!! \n" ); document.write( "

Algebra.Com's Answer #653949 by ikleyn(52832) $\"\"$ $\"About$

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\n" ); document.write( "use the cosine of a sum and cosine of a difference identities to find cos(s+t) and cos(s-t).\r
\n" ); document.write( "\n" ); document.write( "sin s= -4/5 in Q IV
\n" ); document.write( "sin t= 12/13 in Q II\r
\n" ); document.write( "\n" ); document.write( "what is cos(s+t)?
\n" ); document.write( "what is cos(s-t)?\r
\n" ); document.write( "\n" ); document.write( "thank you in advance!!!
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document.write( "Use the formulas \r\n" );
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document.write( "cos(s+t) = cos(s)*cos(t) - sin(s)*sin(t)    (1)\r\n" );
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document.write( "   and\r\n" );
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document.write( "cos(s-t) = cos(s)*cos(t) + sin(s)*sin(t)    (2)\r\n" );
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document.write( "Regarding these formulas, see the lesson Addition and subtraction formulas in this site.\r\n" );
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document.write( "In addition to the given  sin(s) =   and  sin(t) = , you need to know  cos(s) and cos(t).\r\n" );
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document.write( "1.  cos(s) =  =  =  =  =  = .\r\n" );
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document.write( "   The sign \"+\" was chosen for the square root because cos(s) is positive in Q4.\r\n" );
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document.write( "2.  cos(t) = = - = - = - = - = - =.\r\n" );
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document.write( "   The sign \"-\" was chosen for the square root because cos(t) is negative in Q2.\r\n" );
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document.write( "Now all you need to do is to substitute everything into the formulas (1) and (2) and make the calculations.\r\n" );
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document.write( "cos(s-t) =  =  =  = ,   and\r\n" );
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document.write( "cos(s+t) =  =  =  = .\r\n" );
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\n" ); document.write( "\n" ); document.write( "Solved.\r
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