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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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    (Name of Person(s) Filing Proxy Statement, if other than the Registrant)
 
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                       [Mentor Graphics Corporation Logo]
 
                                                               December 28, 1998
 
                        TIME IS SHORT--EVERY VOTE COUNTS
 
DEAR QUICKTURN STOCKHOLDER:
 
    Today, we increased the price of our tender offer to $14 per share matching
in cash the proposed amount to be paid in Cadence stock pursuant to the merger
proposal announced by Quickturn on December 9, 1998.
 
    In order to avoid triggering Quickturn's poison pill, we are forced to
reduce the number of shares to be purchased in the first step tender offer to
2,100,000 shares. These shares, together with the 591,500 shares that we already
own, would represent 14.9% of Quickturn's outstanding stock.
 
                  MAXIMIZE THE VALUE OF YOUR QUICKTURN SHARES
                    VOTE THE GOLD STRIPED PROXY CARD TODAY!
 
    If you vote to replace the current Quickturn directors at the January 8,
1999 Special Meeting, we will encourage the new directors, subject to their
fiduciary duties and Quickturn's rights and obligations under the Cadence merger
agreement, TO SEEK TO PROMPTLY AUCTION QUICKTURN TO THE HIGHEST BIDDER -- BE IT
MENTOR, CADENCE OR ANY OTHER COMPANY.
 
  -  In our opinion, the current Quickturn directors once again breached their
     fiduciary duties to you and your fellow stockholders WHEN THEY FAILED TO
     SEE IF MENTOR OR ANYONE ELSE WAS WILLING TO PAY MORE IN A FAIR AUCTION AND
     INSTEAD ATTEMPTED TO LOCKUP A $14 STOCK DEAL WITH CADENCE WITH EXCESSIVE
     AND UNJUSTIFIED BREAKUP FEES AND STOCK OPTIONS.
 
  -  IN OUR OPINION, IT WAS CLEARLY NOT IN YOUR BEST INTERESTS TO LOCK UP A DEAL
     WITH CADENCE AND SIGNIFICANTLY HINDER THE BIDDING PROCESS BY AGREEING TO
     PAY CADENCE A POTENTIAL $17.5 MILLION BREAKUP FEE -- WITHOUT EVEN WAITING
     TO RECEIVE MENTOR'S REVISED OFFER.
 
  -  We believe Mentor should have been given the same opportunity as Cadence to
     conduct due diligence -- IF THE CURRENT DIRECTORS REALLY WANTED TO GET YOU
     THE BEST PRICE FOR YOUR SHARES.
 
             MENTOR WANTS TO BUY ALL OF QUICKTURN -- BUT WE WANT A
                 LEVEL PLAYING FIELD -- NOT ONE THAT IS STACKED
                      BY QUICKTURN'S INCUMBENT DIRECTORS!
 
    Remember, for more than four months, we have been trying to get the current
board to sit down with us and negotiate a friendly deal in the best interests of
all stockholders. The Quickturn directors in our opinion hastily signed up the
Cadence stock deal to again attempt to defeat Mentor's all cash tender offer --
LESS THAN ONE WEEK AFTER THE DELAWARE COURT OF CHANCERY RULED THAT QUICKTURN'S
DIRECTORS BREACHED THEIR FIDUCIARY DUTIES TO STOCKHOLDERS BY ADOPTING THEIR
POISON PILL AMENDMENTS.

    It is now clearer to us than ever before that it is necessary to replace the
Quickturn directors with our nominees if you want Quickturn sold for the highest
price possible.
 
    Following the completion of our tender offer, we will seek to consummate a
negotiated merger in which all other shares would be acquired for $14 cash per
share, plus an additional amount equal to approximately 75% of the $17.5 million
breakup fee that Quickturn's directors agreed to pay Cadence, if we are able to
invalidate these exorbitant fees. WE THINK THIS BREAKUP FEE IS EXCESSIVE, NOT
JUSTIFIED AND YET ANOTHER BREACH BY QUICKTURN'S DIRECTORS OF THEIR FIDUCIARY
DUTIES TO QUICKTURN'S STOCKHOLDERS. Stockholders would share with Mentor this
payment, to the extent we are successful in invalidating it, through our new
lawsuit against Quickturn's directors for breaching their fiduciary duties in
approved the Cadence merger agreement.
 
    Furthermore, the precise timing of their deal with Cadence, in our view,
shows clearly that THEY WERE MORE INTERESTED IN PROTECTING THEIR CONSULTING
AGREEMENTS AND THE JOBS OF KEITH LOBO AND OTHERS -- INSTEAD OF OBTAINING THE
HIGHEST AND BEST PRICE FOR YOU AND YOUR FELLOW QUICKTURN STOCKHOLDERS. THE FACTS
ARE:
 
  -  On December 8, 1998, Quickturn invited Mentor to consider submitting a
     revised offer to acquire Quickturn, and Mentor said it would respond by
     5:00 p.m. on December 9, 1998. Despite Quickturn's investment banker
     statement at that time that this timing was acceptable -- THEY WENT AHEAD
     AND LOCKED UP THE CADENCE DEAL.
 
  -  We believe they again breached their fiduciary duties to Quickturn
     stockholders by ignoring this prompt and positive indication from Mentor,
     and instead proceeding to approve the Cadence merger agreement on December
     8 -- WITHOUT MAKING EVEN ANOTHER CALL TO MENTOR OR EVEN WAITING 24 HOURS TO
     RECEIVE MENTOR'S REVISED OFFER.
 
  -  Quickturn formally signed its $14 per share Cadence stock merger agreement
     with its exorbitant breakup fees on the evening of December 8 -- IN OUR
     OPINION TO PREVENT A HIGHER OFFER FROM MENTOR.
 
  WE BELIEVE THE QUICKTURN DIRECTORS HAVE ONCE AGAIN BREACHED THEIR FIDUCIARY
DUTIES BY FAILING TO SEEK TO MAXIMIZE THE PRICE YOU WOULD RECEIVE FROM A SALE OF
                                   QUICKTURN
 
    We have reiterated our willingness, as expressed in our December 15 letter
to Keith R. Lobo, Quickturn's President, that Mentor is prepared to pay more for
Quickturn in a negotiated merger than the $14 per share proposed in Cadence's
all stock deal. As we indicated at that time, the final amount over $14 to be
paid under Mentor's negotiated merger proposal will be influenced by due
diligence demonstrating greater value in Quickturn and the extent of the
invalidation of the $17.5 million in breakup fees for Cadence. QUICKTURN'S
DIRECTORS IN ESSENCE DIVERTED NEARLY $1 PER SHARE TO CADENCE RATHER THAN TO
QUICKTURN STOCKHOLDERS.
 
    REMEMBER, THIS IS NOT THE FIRST TIME MENTOR HAS INDICATED TO QUICKTURN THAT
IT WOULD BE WILLING TO INCREASE THE PURCHASE PRICE FOR YOUR SHARES, IF THE
QUICKTURN DIRECTORS WOULD SIMPLY SIT DOWN WITH US, NEGOTIATE AND SHOW US GREATER
VALUES.

               DON'T LET THE CURRENT QUICKTURN BOARD CONTINUE TO
                    MAKE YOUR INVESTMENT DECISIONS FOR YOU!
 
    The Quickturn directors continue to take actions that attempt to prevent
Quickturn stockholders from maximizing the value of their investment. The
current board does not appear to remember that less than three weeks ago the
Delaware Court of Chancery found that they violated their fiduciary duties to
the Quickturn stockholders in an attempt to defeat Mentor's all cash offer by
amending Quickturn's poison pill.
 
                REPLACE THE CURRENT QUICKTURN BOARD OF DIRECTORS
                 WITH MENTOR'S NOMINEES WHO UNDERSTAND THAT THE
             STOCKHOLDERS -- NOT THE DIRECTORS -- OWN THE COMPANY!
 
    We urge you to replace the current Board of Directors with the Mentor
nominees who, subject to their fiduciary responsibilities and Quickturn's rights
and obligations under the Cadence merger agreement, WILL TAKE THE NECESSARY
STEPS TO MAXIMIZE STOCKHOLDER VALUE. If you vote to replace the current
Quickturn directors, Mentor would encourage the new directors TO SELL QUICKTURN
IN A PROMPT AUCTION TO THE HIGHEST BIDDER.
 
    EXERCISE YOUR RIGHTS AS STOCKHOLDERS BY SIGNING AND RETURNING THE ENCLOSED
GOLD STRIPED PROXY CARD TODAY.
 
    Even if you have already tendered your shares or previously voted you should
vote your enclosed proxy to be sure to have your vote counted.
 
                  TENDERING YOUR SHARES ALONE WILL NOT REPLACE
                        THE CURRENT QUICKTURN DIRECTORS!
 
    If you have any questions, or require assistance, please call MacKenzie
Partners, Inc. at (800) 322-2885 TOLL FREE or (212) 929-5500 COLLECT.
 
                                           Sincerely,
                                           /s/ Walden C. Rhines
 
                                           Dr. Walden C. Rhines
                                           President and Chief Executive Officer

    If you have any questions or need assistance in completing the GOLD STRIPED
proxy card, please contact:
 
                           [MacKenzie Partners, Inc.]
 
                                156 Fifth Avenue
                            New York, New York 10010
           CALL TOLL-FREE (800) 322-2885 or (212) 929-5500 (collect)