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colakumarfanta
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a9=x^13* y^21 since both are odd power if anyone is -ve then a9 is<0
s1) insuff as x>0---> x^13>0 but we need to know y also
s2) suff as xy<0 ---->if x<0, then y>0 and if x>0 then y<0 ,and in both the cases x^13y^21 will be a -ve no.hence suff
