## Precalculus (6th Edition) Blitzer

The provided statement does not make complete sense because using the sum and difference formulas one can truly calculate the exact value for the known angles like $15{}^\circ ,30{}^\circ ,45{}^\circ ,60{}^\circ ,105{}^\circ$. But suppose there is a need to find the value of $\sin \left( 60{}^\circ +1{}^\circ \right)\text{or}\sin \left( 90{}^\circ -29{}^\circ \right)$ but the exact value of the trigonometric functions for $1{}^\circ \text{or }29{}^\circ$ is not exactly known. Hence, the exact value cannot be calculated.