Question

Question 18

Square of $9x-7xy$ is
a) $81x^2+49x^2{y}^2$
b) $81x^2-49x^2{y}^2$
c) $81x^2+49x^2{y}^2-126x^{2}y$
d) $81x^2+49x^2{y}^2-63x^{2}y$

Answer 1

c) $81x^2+49x^2{y}^2-126x^{2}y$
$\to$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, $(a-b{)}^2=a^2-2ab+b^2$]
$=81x^2-126x^{2}y+49x^2{y}^2=81x^2+49x^2{y}^2-126x^{2}y$