Verifying Identities
In this section, we will look at strategies to verify identities.
- Try working on the more complicated side of the identity.
- Use the Reciprocal and Quotient Identities to write functions on one side of the identity in terms of the functions on the other side of the identity. Simplify the resulting complex fractions.
- Add rational expressions with unlike denominators by obtaining common denominators.
- Use the Pythagorean Identities to “exchange” sines and cosines, secants and tangents, cosecants and cotangents; and to simplify sums or differences of squares to one term.
- Multiply the numerator and denominator by Pythagorean Conjugates to use the Pythagorean Identities.
- If you find yourself stuck working with one side of the identity, try starting with the other side of the identity and see if you can find a way to bridge the two parts of your work.
We are now at a difficult point. The equation does not seem to be simplifying and we are not making any progress. It is possible that this is not an identity. To prove that we have to go back and use a test value in our original equation. Let’s try . Since this is a trig value, we can go ahead and evaluate it. We again find a common denominator With a little simplifying, we have the following equation. is definitely larger than (verify with a calculator) and thus not equal. We have proved that this equation is not an identity.