Question

The value of $9x^2+49y^2-42xy$ when $x=15$ and $y=3$ is ___________.

• A. $636$
• B. $576$
• C. $456$
• D. $386$

Answer 1

The correct option is B $576$

Given expression is $f(x,y)=9x^2+49y^2-42xy$

We need to find value of expression at $x=15,y=3$

$f(15,3)=[9\times (15{)}^2]+[49\times \left(3{)}^2\right]-[42\times 15\times 3]$

$=[9\times 225]+[49\times 9]-[42\times 45]$

$=2025+441-1890$

$=576$