Prove $\cos \left( x + \frac { \pi } { 4 } \right) + \cos \left( x - \frac { \pi } { 4 } \right) = \sqrt { 2 } \cos x$

question 83

Essay

Prove $\cos \left( x + \frac { \pi } { 4 } \right) + \cos \left( x - \frac { \pi } { 4 } \right) = \sqrt { 2 } \cos x$
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Definitions:

Payor Bank

A bank that is responsible for paying a check or draft drawn upon it by its customer.

Certified Checks

Checks guaranteed by a bank, verifying that there is sufficient funds available in the issuer's account to cover the check.

Teller's Checks

A bank-issued check drawn on the bank's own funds and signed by a teller, typically used when someone needs to guarantee payment.

Refuse

To decline to accept or to reject.