Answer:
The salutation for a business letter should always be formal, even if the business letter is to someone you know well. Examples:
Dear Sir/Madam,
Dear Mr. Shepard,
Dear Ms. Huang,
Dear Prof. Sampson,
Sales Manager,
To Whom It May Concern,
The close of a business letter should always be formal as well:
Sincerely,
Very Truly Yours,
Most Sincerely,
Yours Truly,
The salutation for a personal letter is determined by your relationship with the recipient of the letter and can be formal or very casual. Examples:
Hi Mom,
Dear Aunt Anna,
Hey Rollie,
Sweetie,
Dear Friends,
Fred and Ginger,
The close of a personal letter can also be formal or informal. You can close with words much like you would use if you were speaking to that person.