no ... I said the rectangle's area equals 4 congruent triangle areas. The area of one triangle is $\displaystyle \frac{1}{2} \cdot 5\cos{\theta} \cdot 5\sin{\theta}$. Multiplying this expression by four gives the rectangle area, $\displaystyle A = 25\sin(2\theta)$.

Note that the maximum area of the rectangle is 25 because the largest value that $\displaystyle \sin(2\theta)$ can be is 1.

$\displaystyle A_{max} = 25 \cdot [the \, largest \, possible \, value \, of \, \sin(2\theta)] = 25(1) = 25$