(1+123+234)×(123+234+345)-(1+123+234+345)×(123+234)

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(1+123+234)×(123+234+345)-(1+123+234+345)×(123+234)
(1+123+234)×(123+234+345)-(1+123+234+345)×(123+234)

(1+123+234)×(123+234+345)-(1+123+234+345)×(123+234)
设123+234为a,
则原式=(1+a)(345+a)-(346+a)a
=345+a+345a+a平方-346a-a平方
=345