Question
Expand : $(3x - 5y - 2z) (3x - 5y + 2z)$
✓
Answer
$(3x-5y-2z) (3x-5y+2z)$
$= {(3x-5y)-(2z)} {(3x-5y)+(2z)}$
$= (3x-5y)^2-(2z)^2{since(a+b) (a-b)=a^2-b^2}$
$= 9x^2+25y^2- 2 \times 3x\times 5y-4z^2$
$= 9x^2+25y^2-30xy-4z^2$
$= 9x^2+25y^2-4z^2-30xy$
$= {(3x-5y)-(2z)} {(3x-5y)+(2z)}$
$= (3x-5y)^2-(2z)^2{since(a+b) (a-b)=a^2-b^2}$
$= 9x^2+25y^2- 2 \times 3x\times 5y-4z^2$
$= 9x^2+25y^2-30xy-4z^2$
$= 9x^2+25y^2-4z^2-30xy$
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