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                                QUICKTURN DESIGN SYSTEMS, INC.
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                (Name of Registrant as Specified In Its Charter)

                                 MENTOR GRAPHICS CORPORATION
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                       [Mentor Graphics Corporation Logo]
 
                                                            December 28, 1998
 
Mr. Keith R. Lobo
President and Chief Executive Officer
Quickturn Design Systems, Inc.
55 West Trimble Road
San Jose, California 96131
 
Dear Keith:
 
    As you should know by now, Mentor today increased its offering price for
shares of Quickturn Design Systems, Inc. to $14.00 cash per share from $12.125
cash per share, and changed the number of shares being sought to 2,100,000
shares. Those shares, together with the 591,500 shares of Quickturn stock we
already own, will result in Mentor owning a total of 14.9% of Quickturn's
outstanding stock, the maximum amount that can be acquired without triggering
Quickturn's poison pill.
 
    We have the cash in hand to consummate our tender offer for the 2,100,000
shares, and we believe the financing commitments we have in place would allow us
to complete our proposed second-step merger.
 
    The purpose of the offer is to add to our equity interest in Quickturn as
the first step in completing a merger agreement or a similar business
combination of our two companies. The next logical step would be the negotiation
of a merger agreement between Mentor and Quickturn subsequent to which we would
acquire the balance of the Quickturn shares at the same cash price as that paid
in the tender offer. While we believe the extraordinarily restrictive no-shop
provisions of your merger agreement with Cadence are not legal, and are suing in
Delaware Court of Chancery to invalidate them, we stand ready to consider
increasing our offer price and the price to be paid per share in a negotiated
merger transaction if negotiation and due diligence demonstrate greater value of
Quickturn to Mentor.
 
    As evidence of our commitment to completing a merger, we would pay Quickturn
stockholders, in addition to $14.00 per share, a portion of the $17.5 million
break-up fee that is part of the current agreement with Cadence, in the event
that we are successful in invalidating this and other provisions through either
the present litigation in the Delaware Court of Chancery or a negotiated merger
agreement.
 
    Now that we have increased our offer--and consistent with your rights under
the Cadence merger agreement--it is time for you to sit down with us, negotiate
and fulfill your fiduciary duty to Quickturn's stockholders.
 
    If, however, you choose not to negotiate with us, your stockholders will be
able to express their outrage at your conduct in less than two weeks. The
special

meeting of Quickturn stockholders will be held on Friday, January 8, 1999 to
consider our proposals relating to your removal and the removal of the rest of
the Quickturn Board.
 
    If our nominees are elected as directors of Quickturn, we would encourage
them, subject to their fiduciary duties as directors under applicable law and in
accordance with Quickturn's rights and obligations under the merger agreement
with Cadence, to seek to auction Quickturn to the highest bidder. We also would
encourage the nominees, subject to their fiduciary duties as directors of
Quickturn under applicable law and in accordance with Quickturn's rights and
obligations under the merger agreement with Cadence, to allow any bidder,
including Mentor, promptly to conduct a due diligence review of Quickturn and to
seek to execute a merger agreement with the highest bidder. We believe that any
merger agreement could be executed within 30 days of the nominees' election as
directors of Quickturn.
 
    It is time for you and the other Quickturn directors to act responsibly and
to maximize value for your stockholders.
 
                                           Very truly yours,
 
                                           /s/ WALDEN C. RHINES
                                           -------------------------------------
                                           President and
                                             Chief Executive Officer