Thread: The Language of Math View Single Post
 10-03-2012, 04:07 PM #11 cj wisty Non Ignorant Math Team   Join Date: Nov 2011 Posts: 2,884 Rep Power: 15 i already knew this but when i saw the OP i wondered why that was and its actually pretty obvious and can b proven simply. if u write all ur numbers in base x (OP says x=10 which is ofc the most commonly used base) and then use timetables of x-1 (OP says x-1=9) and then write out all the timetables u get 1(x-1)=(x-1) obviously cuz x is the base the digit on the left will b 0 and the digit on the right will b (x-1) 2(x-1)=(2x-2) obviously cuz x is the base the digit on the left will b 1 and the digit on the right will b (x-2) 3(x-1)=(3x-3) obviously cuz x is the base the digit on the left will b 2 and the digit on the right will b (x-3) 4(x-1)=(4x-4) obviously cuz x is the base the digit on the left will b 3 and the digit on the right will b (x-4) and this can b repeated for all two digit numbers when multiplying a number which is 1 under the base. so eg if we skip to 7 times 9. so here were saying x=10 and (x-1)=9 and so 7 times 9=7(x-1)=7x-7 which means the left hand digit is 6 and the right hand digit is x-7 so we know the left hand digit is 6 and we now need to find out the right hand digit. right hand digit is (x-7) and substituting (x=10) gives us (10-7) which is 3. so the right hand digit is 3. putting these 2 facts together gives us the number 63. therefore 7 times 9=63 __________________ NON-IGS