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is ST <0

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by raviki8208 » Tue May 25, 2010 5:04 am
colakumarfanta wrote:is ST <0 ?
(1) S ^ 3 * T ^ 5 <0 (2) S ^ 8 * T ^ 9> 0

using 1 alone

S ^ 3 * T ^ 5 <0
here both powers are odd so, either s or t is -ve. so st is -ve -- sufficient

using 2 alone

S ^ 8 * T ^ 9>0
t is +ve ad its power is even
But we can not comment on s so, insufficient


My answer is A
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by thephoenix » Wed May 26, 2010 8:13 pm
colakumarfanta wrote:is ST <0 ?
(1) S ^ 3 * T ^ 5 <0 (2) S ^ 8 * T ^ 9> 0
1)
s^3 * t^5<0---->s^3<0 and t^5>0 or s^3> 0and T^5<0[because odd power does not changes the sign]
---->s<0;t>0 or s>0;t<0--->st<0 suff

2)
SINCE t^9 is always >0 but s san be >0 or <0
insuff
A
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