document.write( "Question 634796: Hi, I've been stuck on this one for a considerable amount of time.
\n" ); document.write( "=> Show that the equation 4cos(2x)-3sin(x)csc(x)^3+6 = 0, can be expressed as
\n" ); document.write( "8y^4-10y^2+3=0 if sin(x) = y
\n" ); document.write( "Hence, solve the equation for x\r
\n" ); document.write( "\n" ); document.write( "Thankyou for your help
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Algebra.Com's Answer #399927 by jsmallt9(3758)\"\" \"About 
You can put this solution on YOUR website!
\"4cos%282x%29-3sin%28x%29csc%5E3%28x%29%2B6+=+0\"
\n" ); document.write( "As the problem suggest, you want everything expressed in terms of sin(x). SO we will start by substituting for the \"cos%282x%29\" and \"csc%5E3%28x%29\". The csc is easy. csc is the reciprocal of sin. So \"csc%5E3%28x%29+=+1%2Fsin%5E3%28x%29\". We have three variations for cos(2x):
  • \"cos%282x%29+=+cos%5E2%28x%29-sin%5E2%28x%29\"
  • \"cos%282x%29+=+2cos%5E2%28x%29-1\"
  • \"cos%282x%29+=+1-2sin%5E2%28x%29\"
Since we want everything in terms of sin(x), we will use the third variation. Substituting in we get:
\n" ); document.write( "\"4%281-2sin%5E2%28x%29%29-3sin%28x%29%281%2Fsin%5E3%28x%29%29%2B6+=+0\"
\n" ); document.write( "Note the use of parentheses! These are important when making substitutions. Simplifying we get:
\n" ); document.write( "\"4-8sin%5E2%28x%29-%283%2Fsin%5E2%28x%29%29%2B6+=+0\"
\n" ); document.write( "\"-8sin%5E2%28x%29-%283%2Fsin%5E2%28x%29%29%2B10+=+0\"

\n" ); document.write( "Now that everything is in terms of sin(x), the next task is to get rid of the fraction. Multiplying both sides by \"sin%5E2%28x%29\" we get:
\n" ); document.write( "\"-8sin%5E4%28x%29-3%2B10sin%5E2%28x%29+=+0\"
\n" ); document.write( "As you may see, we're getting close. We can get the signs right if we multiply both sides by -1:
\n" ); document.write( "\"8sin%5E4%28x%29%2B3-10sin%5E2%28x%29+=+0\"
\n" ); document.write( "And rearrange the terms:
\n" ); document.write( "\"8sin%5E4%28x%29-10sin%5E2%28x%29%2B3+=+0\"

\n" ); document.write( "Now we can substitute \"y\" for \"sin(x)\":
\n" ); document.write( "\"8y%5E4-10y%5E2%2B3+=+0\"
\n" ); document.write( "The reason we made this substitution is that it is easier to see how to solve
\n" ); document.write( "\"8y%5E4-10y%5E2%2B3+=+0\"
\n" ); document.write( "than it is to see how to solve
\n" ); document.write( "\"8sin%5E4%28x%29-10sin%5E2%28x%29%2B3+=+0\"
\n" ); document.write( "(Eventually you will see how to solve without making the substitution.) Note: An even better substitution is \"z+=+sin%5E2%28x%29\" This would make the substituted equation:
\n" ); document.write( "\"8z%5E2-10z%2B3=0\"
\n" ); document.write( "which I think you would agree is even easier to solve than:
\n" ); document.write( "\"8y%5E4-10y%5E2%2B3+=+0\"
\n" ); document.write( "But we will go with the substitution your teacher/book suggested.
\n" ); document.write( "

\n" ); document.write( "\"8y%5E4-10y%5E2%2B3+=+0\"
\n" ); document.write( "is an equation in what is called quadratic form. (A \"pure\" quadratic would have y-squared and y terms.) We can solve it with the same methods used to solve pure quadratics. One of these is factoring:
\n" ); document.write( "\"%284y%5E2-3%29%282y%5E2-1%29+=+0\"
\n" ); document.write( "If this factoring is not immediately obvious to you, then look at it for a while and let it sink in. Multiply it back out and see if you get \"8y%5E4-10y%5E2%2B3+=+0\". Look back at our alternate substitution, \"8z%5E2-10z%2B3=0\", and think about how that would factor. It might help you see how we got the factors above.

\n" ); document.write( "Now we use the Zero Product Property:
\n" ); document.write( "\"4y%5E2-3+=+0\" or \"2y%5E2-1=0\"
\n" ); document.write( "Solving these...
\n" ); document.write( "\"4y%5E2+=+3\" or \"2y%5E2=+1\"
\n" ); document.write( "\"y%5E2+=+3%2F4\" or \"y%5E2=+1%2F2\"
\n" ); document.write( "Finding the square root of each side (and not forgetting about the negative square roots) we get:
\n" ); document.write( "\"y+=+sqrt%283%2F4%29\" or \"y+=+-sqrt%283%2F4%29\" or \"y+=+sqrt%281%2F2%29\" or \"y+=+-sqrt%281%2F2%29\"
\n" ); document.write( "Rationalizing the denominators...
\n" ); document.write( "\"y+=+sqrt%283%29%2Fsqrt%284%29\" or \"y+=+-sqrt%283%29%2Fsqrt%284%29\" or \"y+=+sqrt%28%281%2F2%29%282%2F2%29%29\" or \"y+=+-sqrt%28%281%2F2%29%282%2F2%29%29\"
\n" ); document.write( "\"y+=+sqrt%283%29%2F2\" or \"y+=+-sqrt%283%29%2F2\" or \"y+=+sqrt%282%2F4%29\" or \"y+=+-sqrt%282%2F4%29\"
\n" ); document.write( "\"y+=+sqrt%283%29%2F2\" or \"y+=+-sqrt%283%29%2F2\" or \"y+=+sqrt%282%29%2Fsqrt%284%29\" or \"y+=+-sqrt%282%29%2Fsqrt%284%29\"
\n" ); document.write( "\"y+=+sqrt%283%29%2F2\" or \"y+=+-sqrt%283%29%2F2\" or \"y+=+sqrt%282%29%2F2\" or \"y+=+-sqrt%282%29%2F2\"

\n" ); document.write( "We've solved for y. But we really want to solve for x. So it is time to substitute sin(x) back in for the y:
\n" ); document.write( "\"sin%28x%29+=+sqrt%283%29%2F2\" or \"sin%28x%29+=+-sqrt%283%29%2F2\" or \"sin%28x%29+=+sqrt%282%29%2F2\" or \"sin%28x%29+=+-sqrt%282%29%2F2\"
\n" ); document.write( "All of these are special angle sin's so every x is a special angle (so put your calculator away).

\n" ); document.write( "We should recognize that the reference angle for both \"sin%28x%29+=+sqrt%283%29%2F2\" or \"sin%28x%29+=+-sqrt%283%29%2F2\" will be \"pi%2F3\". And since we have both positive and negative \"sqrt%283%29%2F2\", x will terminate in all four quadrants. So for \"sin%28x%29+=+sqrt%283%29%2F2\" or \"sin%28x%29+=+-sqrt%283%29%2F2\":
\n" ); document.write( "\"x+=+pi%2F3+%2B+2pi%2An\" (1st quadrant, \"sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+pi+-+pi%2F3+%2B+2pi%2An\" (2nd quadrant, \"sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+pi+%2B+pi%2F3+%2B+2pi%2An\" (3rd quadrant, \"-sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+-pi%2F3+%2B+2pi%2An\" (4th quadrant, \"-sqrt%283%29%2F2\")
\n" ); document.write( "with the middle two equations simplifying to:
\n" ); document.write( "\"x+=+pi%2F3+%2B+2pi%2An\" (1st quadrant, \"sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+2pi%2F3+%2B+2pi%2An\" (2nd quadrant, \"sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+4pi%2F3+%2B+2pi%2An\" (3rd quadrant, \"-sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+-pi%2F3+%2B+2pi%2An\" (4th quadrant, \"-sqrt%283%29%2F2\")

\n" ); document.write( "For \"sin%28x%29+=+sqrt%282%29%2F2\" and \"sin%28x%29+=+-sqrt%282%29%2F2\" we should recognize that the reference angle will be \"pi%2F4\" and x will again terminate in all four quadrants (since we have both positive and negative \"+sqrt%282%29%2F2\". This gives us:
\n" ); document.write( "\"x+=+pi%2F4+%2B+2pi%2An\" (1st quadrant, \"sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+pi+-+pi%2F4+%2B+2pi%2An\" (2nd quadrant, \"sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+pi+%2B+pi%2F4+%2B+2pi%2An\" (3rd quadrant, \"-sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+-pi%2F4+%2B+2pi%2An\" (4th quadrant, \"-sqrt%282%29%2F2\")
\n" ); document.write( "with the middle two equations simplifying to:
\n" ); document.write( "\"x+=+pi%2F4+%2B+2pi%2An\" (1st quadrant, \"sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+3pi%2F4+%2B+2pi%2An\" (2nd quadrant, \"sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+5pi%2F4+%2B+2pi%2An\" (3rd quadrant, \"-sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+-pi%2F4+%2B+2pi%2An\" (4th quadrant, \"-sqrt%282%29%2F2\")

\n" ); document.write( "The full general solution to your equation is all 8 of the equations above:
\n" ); document.write( "\"x+=+pi%2F3+%2B+2pi%2An\" (1st quadrant, \"sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+2pi%2F3+%2B+2pi%2An\" (2nd quadrant, \"sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+4pi%2F3+%2B+2pi%2An\" (3rd quadrant, \"-sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+-pi%2F3+%2B+2pi%2An\" (4th quadrant, \"-sqrt%283%29%2F2\")
\n" ); document.write( "\"x+=+pi%2F4+%2B+2pi%2An\" (1st quadrant, \"sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+3pi%2F4+%2B+2pi%2An\" (2nd quadrant, \"sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+5pi%2F4+%2B+2pi%2An\" (3rd quadrant, \"-sqrt%282%29%2F2\")
\n" ); document.write( "\"x+=+-pi%2F4+%2B+2pi%2An\" (4th quadrant, \"-sqrt%282%29%2F2\")
\n" ); document.write( "
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