document.write( "Question 925472: How do I evaluate these this two sine and cosine problems. Difficult!?
\n" ); document.write( "Evaluate cos(a − b) if cos a = 4/5 tan a < 0, tan b = −√15 and cos b < 0\r
\n" ); document.write( "\n" ); document.write( "cos(a-b) =\r
\n" ); document.write( "\n" ); document.write( "and\r
\n" ); document.write( "\n" ); document.write( "Evaluate sin(x + y) if sin x = 3/5 sec x > 0, cos y = − (2√5)/5 and tan y < 0.\r
\n" ); document.write( "\n" ); document.write( "sin(x + y) =\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "THANKS! \n" ); document.write( "

Algebra.Com's Answer #561554 by richard1234(7193) $\"\"$ $\"About$

You can put this solution on YOUR website!
Not too difficult - use cosine addition/subtraction formulas:\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "cos(a-b) = cos a cos b - sin a sin b\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "You know cos a and the sign of tan a, so you have enough information to find sin a (hint: sin^2 a + cos^2 a = 1). You know tan b and the sign of cos b, which is enough to determine sin b and cos b (hint: also use sin^2 b + cos^2 b = 1).\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "The second problem is similar, except you will be using the identity sin(x+y) = sin x cos y + sin y cos x. \n" ); document.write( "

\n" );