document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=706424>Question 706424</A>: <I><SPAN STYLE=\"word-break: break-all;\"> A rock is thrown straight up into the air from a height of 4 feet, the height of the rock above the ground , in feet, t seconds after it is thrown is given by -16t^2 + 56T + 4. for how many seconds will the height of the rock be at least 28 feet above the ground? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #435160 by </B><A HREF=/tutors/aboutme.mpl?userid=nerdybill><B>nerdybill(7384)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=nerdybill><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/cgi-bin/show-face.mpl?user=nerdybill><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=435160\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> h = -16t^2 + 56T + 4<BR>\n" );
document.write( "set h to 28 and solve for t:<BR>\n" );
document.write( "28 = -16t^2 + 56T + 4<BR>\n" );
document.write( "-28 = 16t^2 - 56T - 4<BR>\n" );
document.write( "0 = 16t^2 - 56T + 24<BR>\n" );
document.write( "0 = 2t^2 - 7T + 3<BR>\n" );
document.write( "factoring the left:<BR>\n" );
document.write( "0 = (2t-1)(t-3)<BR>\n" );
document.write( "t = {.5, 3}<BR>\n" );
document.write( "Time in above 28 feet then:<BR>\n" );
document.write( "3 - .5 = 2.5 seconds </SPAN>\n" );
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