Square of 9x−7xy is
a) 81x2+49x2y2
b) 81x2−49x2y2
c) 81x2+49x2y2−126x2y
d) 81x2+49x2y2−63x2y
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Solution
c) 81x2+49x2y2−126x2y →Square of (9x−7xy)=(9x−7xy)2
Comparing with (a−b)2, we get a=9x and b=7xy (9x−7xy)2=(9x)2−2.9x.7xy+(7xy)2 [using the identity, (a−b)2=a2−2ab+b2] =81x2−126x2y+49x2y2=81x2+49x2y2−126x2y