document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=565431>Question 565431</A>: <I><SPAN STYLE=\"word-break: break-all;\"> A ball is launched from the ground straight up into the air at a rate of 80 feet per second. Its height h above the ground (in feet) after t seconds is h = 80t – 16t2. After how many seconds is the ball 80 feet high on the way back down? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #365793 by </B><A HREF=/tutors/aboutme.mpl?userid=htmentor><B>htmentor(1343)</B></A><img src=\"/images/stars/stars-12.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=htmentor><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=htmentor><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=365793\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> A ball is launched from the ground straight up into the air at a rate of 80 feet per second. Its height h above the ground (in feet) after t seconds is h = 80t – 16t2. After how many seconds is the ball 80 feet high on the way back down?<BR>\n" );
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document.write( "Substituting h=80 and putting in standard form gives:<BR>\n" );
document.write( "-16t^2 + 80t - 80 = 0<BR>\n" );
document.write( "Factor:<BR>\n" );
document.write( "-16(t^2 - 5t + 5) = 0<BR>\n" );
document.write( "t^2 - 5t + 5 = 0<BR>\n" );
document.write( "Solve for t using the quadratic formula:<BR>\n" );
document.write( "t = (5 +- sqrt(25 - 20))/2<BR>\n" );
document.write( "Since we are looking for the time on the way back down, take the larger value:<BR>\n" );
document.write( "t = (5 + sqrt(5))/2 = 3.618 seconds </SPAN>\n" );
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