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

CBSE Class 10 CBSE Class 10 Maths
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If a-b = 7 , a2 + b2 = 85, find a3 - b3 ?

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begin mathsize 16px style straight a minus straight b equals 7 rightwards double arrow left parenthesis straight a minus straight b right parenthesis squared equals 7 squared rightwards double arrow straight a squared plus straight b squared minus 2 ab equals 49 rightwards double arrow left parenthesis straight a squared plus straight b squared right parenthesis minus 2 ab equals 49 rightwards double arrow 85 minus 2 ab equals 49 rightwards double arrow 2 ab equals 36 rightwards double arrow ab equals 18 Now comma straight a cubed minus straight b cubed equals left parenthesis straight a minus straight b right parenthesis cubed plus 3 ab left parenthesis straight a minus straight b right parenthesis space space space space space space space space space space space space equals left parenthesis 7 right parenthesis cubed plus 3 cross times 18 cross times 7 space space space space space space space space space space space space equals 343 plus 378 space space space space space space space space space space space space equals 721 end style