document.write( "Question 602898: If tan A = 9/40, A is in the first quadrant, cos B = -4/5, B is in the third quadrant, find the value of cos(A+B)\r
\n" ); document.write( "\n" ); document.write( "Please help me solve this problem. It's from the homework given to us today, which I have no chance of understanding since I missed class per the doctor's orders, and my friend's notes aren't really that readable...
\n" ); document.write( "If you can, please show me the steps as well. It would really help me understand this lesson. Thank you! \n" ); document.write( "
Algebra.Com's Answer #380483 by Alan3354(69443)\"\" \"About 
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If tan A = 9/40, A is in the first quadrant, cos B = -4/5, B is in the third quadrant, find the value of cos(A+B)
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\n" ); document.write( "Find the cos(A)
\n" ); document.write( "Tan = y/x
\n" ); document.write( "Plot the point (40,9)
\n" ); document.write( "--> hyp = sqrt(40^2 + 9^2) = 41
\n" ); document.write( "cos(A) = x/hyp = 9/41
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\n" ); document.write( "cos(A + B) = cosA*cosB - sinA*sinB (from Wikipedia)
\n" ); document.write( "Find sin(A) and sin(B)
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\n" ); document.write( "sin(A) = y/hyp = 40/41
\n" ); document.write( "sin(B) = -3/5 in Q3 (it's the 3, 4, 5 triangle)
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\n" ); document.write( "cos(A + B) = cosA*cosB - sinA*sinB
\n" ); document.write( "= (9/41)*(-4/5) - (40/41)*(-3/5)
\n" ); document.write( "= -36/205 + 120/205
\n" ); document.write( "= 84/205
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