I. Express the LHS in terms of sine and cosine and simplify
Start from the LHS (left-hand side)
Rewrite $\sec\left(x\right)$ in terms of sine and cosine
II. Express the RHS in terms of sine and cosine and simplify
Start from the RHS (right-hand side)
Combine fractions with different denominator using the formula: $\displaystyle\frac{a}{b}+\frac{c}{d}=\frac{a\cdot d + b\cdot c}{b\cdot d}$
Simplify $\sin\left(x\right)\cos\left(x\right)$ using the trigonometric identity: $\sin(2x)=2\sin(x)\cos(x)$
Divide fractions $\frac{\sin\left(2x\right)\cos\left(x\right)-\cos\left(2x\right)\sin\left(x\right)}{\frac{\sin\left(2x\right)}{2}}$ with Keep, Change, Flip: $a\div \frac{b}{c}=\frac{a}{1}\div\frac{b}{c}=\frac{a}{1}\times\frac{c}{b}=\frac{a\cdot c}{b}$
Multiply the single term $2$ by each term of the polynomial $\left(\sin\left(2x\right)\cos\left(x\right)-\cos\left(2x\right)\sin\left(x\right)\right)$
Using the sine double-angle identity: $\sin\left(2\theta\right)=2\sin\left(\theta\right)\cos\left(\theta\right)$
When multiplying two powers that have the same base ($\cos\left(x\right)$), you can add the exponents
Using the sine double-angle identity: $\sin\left(2\theta\right)=2\sin\left(\theta\right)\cos\left(\theta\right)$
Factor the polynomial $4\sin\left(x\right)\cos\left(x\right)^2-2\cos\left(2x\right)\sin\left(x\right)$ by it's greatest common factor (GCF): $2\sin\left(x\right)$
Simplify the fraction
III. Choose what side of the identity are we going to work on
To prove an identity, we usually begin to work on the side of the equality that seems to be more complicated, or the side that is not expressed in terms of sine and cosine. In this problem, we will choose to work on the right side $\frac{2\cos\left(x\right)^2-\cos\left(2x\right)}{\cos\left(x\right)}$ to reach the left side $\frac{1}{\cos\left(x\right)}$
Apply the trigonometric identity: $\cos\left(2\theta \right)$$=2\cos\left(\theta \right)^2-1$
Simplify the product $-(2\cos\left(x\right)^2-1)$
Multiply $-1$ times $-1$
Cancel like terms $2\cos\left(x\right)^2$ and $-2\cos\left(x\right)^2$
IV. Check if we arrived at the expression we wanted to prove
Since we have reached the expression of our goal, we have proven the identity
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