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CBSE Class 9 Answered

If a-b =7, a2+b2 = 85, find a3 - b3 ?
Asked by nisha_vini29 | 23 Mar, 2017, 04:14: PM
answered-by-expert Expert Answer

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

Answered by | 23 Mar, 2017, 05:04: PM
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