Multiplying together 2-digit numbers in cases where the tens digits added together equal 10 and the ones digits are the same

To multiply together 2-digit numbers in cases where the tens digits added together equal 10 and the ones digits are the same:    ex. 21×81=1701

-Multiply the ones digits of the numbers being multiplied. This yields the last two digits of the answer.     ex. 1×1=_ _01

_Multiply the tens digits of the numbers being multiplied, and to this add the common ones digit of the numbers being multiplied. This yields the first 2 digits of the answer.    ex.  (2×8)+1=17_ _

Prove these:    39×79=3081      42×62=2604      35×75=2625      34×74=2516

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