The common error in proving trigonometric identities is to assume the equality of the left-hand side (LHS) and the right-hand side (RHS) of what is being proved. To prove an identity means to transform the LHS and RHS in the same form.

##### Prove the identity:

###### Solution sure to get 0/5 mark:

This is a ratio so we cross multiply. That is,

So,

Why you get 0 mark? When you cross-multiply, you assume that you have an equation. You assume that the left-hand side is equal to the right-hand side. Isn’t that’s what you are exactly asked to do – To show that they are equal and not assume that they are equal?

##### Solution sure to get 5/5 mark:

Left Hand Side

Right Hand Side

The right hand side is identical to the left hand side hence the identity is proved.