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The product of (x2 + y2) (x2 – y2) is

Since (a + b) (a – b) = a2 – b2 \(\begin{array}{l}\left(x^{2}+y^{2}\right)\left(x^{2}-y^{2}\right)\\=\left(x^{2}\right)^{2}-\left(y^{2}\right)^{2}\\=x^{4}-y^{4}\end{array} \)

Prove that sin [(π / 2) – x] = cos x.

\(\begin{array}{l}\sin \left(\frac{\pi}{2}-\mathrm{x}\right) \\ =\cos \left[\frac{\pi}{2}-\left(\frac{\pi}{2}-\mathrm{x}\right)\right] \\ \cos \left(\frac{\pi}{2}-\mathrm{x}\right)=\sin \mathrm{x} \\ =\cos (-\mathrm{x})=\cos \mathrm{x}\end{array} \)

Solve the equation: (a / 5) + 3 = 2.

The given equation is \(\begin{array}{l}\frac{a}{5}+3=2 \\ \Rightarrow \frac{a}{5}=2-3 \\\Rightarrow \frac{a}{5}=-1 \\ \Rightarrow a=-1 \times 5 \\ \Rightarrow a=-5\end{array} \)