Discussion
The length of a diagonal of a square is 8 cm. If its area (in cm2) is y, then value of (3y - 15) is :
Answer: B
The value of \(3y - 15\) is 81.
The area of a square can be found from its diagonal \(d\) alone, since \(d^{2} = 2a^{2}\):
\[ \begin{aligned} y &= \frac{d^{2}}{2} = \frac{64}{2} = 32\ \text{cm}^{2} \\ 3y - 15 &= 96 - 15 = 81 \end{aligned} \]
96 is \(3y\) before subtracting 15. Treating the diagonal as the side would give an area of 64 and a far larger value, which matches none of the options.
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