December 24, 1998
 
Dear Fellow Quickturn Stockholder:
 
        QUICKTURN ENTERS INTO DEFINITIVE MERGER AGREEMENT WITH CADENCE
 
  On December 8, 1998, the Board of Directors of Quickturn Design Systems,
Inc. unanimously approved a definitive merger agreement with Cadence Design
Systems, Inc., a world leader in electronic design software and services.
Under the Cadence-Quickturn merger agreement, Quickturn will merge with a
wholly owned subsidiary of Cadence in a tax-free stock-for-stock transaction,
and Quickturn stockholders will receive $14 worth of Cadence common stock for
each outstanding share of Quickturn they currently own.
 
  The combination with Cadence will enable Quickturn stockholders to enjoy the
benefits of Cadence's proven business strategy, strong balance sheet and
excellent track record in acquiring and integrating companies. WE BELIEVE THAT
OUR STRATEGIC MERGER WITH CADENCE IS IN THE BEST INTERESTS OF QUICKTURN AND
ITS STOCKHOLDERS.
 
            NEVERTHELESS, MENTOR PERSISTS IN ITS DESPERATE ATTEMPT
                     TO TAKE OVER QUICKTURN AND YOUR BOARD
 
  Despite Quickturn's strategic merger with Cadence, Mentor persists in
soliciting your vote to replace the Quickturn Board with its own handpicked
slate of director nominees. If Mentor's nominees are elected to Quickturn's
Board, their stated primary purpose will be to facilitate Mentor's takeover of
Quickturn at $12.125, an offer your Board determined was inadequate and not in
the best interests of the Company and its stockholders. The Special Meeting of
Quickturn Stockholders that was sought by Mentor Graphics is still scheduled
for January 8, 1999.
 
  YOUR BOARD OF DIRECTORS CONTINUES TO STRONGLY RECOMMEND THAT YOU VOTE
AGAINST MENTOR'S PROPOSALS ON THE ENCLOSED BLUE PROXY CARD. By doing so, you
are sending a strong message to your Board--and to Mentor--that you prefer a
combination of Cadence and Quickturn. If you have previously supported Mentor
by sending in a Gold-striped proxy card, you can still change your vote by
signing, dating and mailing Quickturn's BLUE proxy card today. ONLY THE LATEST
SIGNED AND DATED PROXY CARD IS COUNTED.
 
          REJECT MENTOR'S DESPERATE ATTEMPT TO TAKE OVER YOUR COMPANY
 
         VOTE AGAINST MENTOR'S PROPOSALS ON THE BLUE PROXY CARD TODAY.
 
  Your Board has been working hard on your behalf. Voting the BLUE proxy card
will enable the Board to continue to ensure that your best interests--not
Mentor's--are served.
 
  Enclosed are Quickturn's proxy statement, dated September 21, 1998, and an
amendment to that proxy, dated December 24, 1998. If you have any questions,
or need assistance in voting your shares, please call our proxy solicitor,
Morrow & Co. Inc., toll-free, at 1-800-662-5200.
 
                                          On Behalf of the Board of Directors,
 
                                          /s/ Keith R. Lobo
                                          Keith R. Lobo
                                          Chief Executive Officer and
                                           President