Archive
23rd of March 2017
Mathematics
Q:

If a-b =7, a2+b2 = 85, find a3 - b3 ?

Nisha_vini29 - CBSE - Class IX

Thursday, March 23, 2017 at 16:14:PM

A:

begin mathsize 16px style straight a minus straight b equals 7 space and space straight a squared plus straight b squared equals 85
left parenthesis straight a minus straight b right parenthesis squared equals straight a squared plus straight b squared minus 2 ab
rightwards double arrow left parenthesis 7 right parenthesis squared equals 85 minus 2 ab
rightwards double arrow 49 equals 85 minus 2 ab
rightwards double arrow ab equals 18
So comma space space straight a cubed minus straight b cubed equals left parenthesis straight a minus straight b right parenthesis left parenthesis straight a squared plus straight b squared minus ab right parenthesis
rightwards double arrow straight a cubed minus straight b cubed equals left parenthesis 7 right parenthesis left parenthesis 85 minus 18 right parenthesis
rightwards double arrow straight a cubed minus straight b cubed equals 469 end style

Thursday, March 23, 2017 at 17:04:PM