document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=868372>Question 868372</A>: <I><SPAN STYLE=\"word-break: break-all;\"> If it is known that sin a = 4/5.\r<BR>\n" );
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document.write( "pi/2 < a < pi\r<BR>\n" );
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document.write( "and that sin b = -2sqrt(5)/5\r<BR>\n" );
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document.write( "pi < b < 3pi/2 find the exact value of cos(a + b)\r<BR>\n" );
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document.write( "I am completely lost on this problem, any and all help would be greatly appreciated </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #523812 by </B><A HREF=/tutors/aboutme.mpl?userid=stanbon><B>stanbon(75887)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=stanbon><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/images/nopicture.jpg><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=523812\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> If it is known that sin a = 4/5. <BR>\n" );
document.write( "pi/2 < a < pi ::: QII where x <0 and y > 0<BR>\n" );
document.write( "Since sin = y/r, y = 4 and r = 5<BR>\n" );
document.write( "Then x = -sqrt[25-16] = -3<BR>\n" );
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document.write( "So cos(a) = x/r = -3/5\r<BR>\n" );
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document.write( "and that sin b = -2sqrt(5)/5<BR>\n" );
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document.write( "pi < b < 3pi/2 ::: QIII where x < 0 and y < 0<BR>\n" );
document.write( "Since sin = y/r, y = -2sqrt(5) and r = 5<BR>\n" );
document.write( "Then x = -sqrt[25-20] = -sqrt(5)<BR>\n" );
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document.write( "so cos(b) = -sqrt(5)/5<BR>\n" );
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document.write( "find the exact value of cos(a + b)<BR>\n" );
document.write( "cos(a+b) = cos(a)cos(b)-sin(a)sin(b)<BR>\n" );
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document.write( "= (-3/5)(-sqrt(5)/5) - (4/5)(-2sqrt(5)/5)<BR>\n" );
document.write( "===========================================<BR>\n" );
document.write( "Cheers,<BR>\n" );
document.write( "Stan H.<BR>\n" );
document.write( "================== <BR>\n" );
document.write( " </SPAN>\n" );
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