Question

$Ifco{s}^{4}A–si{n}^{4}A=\frac{3}{5},then2co{s}^{2}A-1$= ?

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

We know $si{n}^2$A + $co{s}^2$A = 1 and $a^2$ - $b^2$ = (a+b) (a-b)

Using the two identities, we have $co{s}^4$A - $si{n}^4$A = ($co{s}^2$A - $si{n}^2$A) ($si{n}^2$A + $co{s}^2$A) = (2 $co{s}^2$A - 1) $\times$ 1 = $\frac{3}{5}$