1.

यदि `a-b=1` और `a^(3)-b^(3)=61` तो `ab` का मान क्या होगा?A. -20B. 20C. 30D. 60

Answer» Correct Answer - B
`a^(3)-b^(3)=61`
`(a-b)(a^(2)+ab+b^(2))=61`
(Given `a-b=1`)
`a^(2)+ab+b^(2)=61`……………i
Now
`a-b=1`
On squaring both side
`a^(2)+b^(2)-2ab=1`
`underbrace(a^(2)+b^(2)ab)-3ab=1`
from equ i
`61-3ab=1`
`3ab=60`
`ab=60/3`
`ab=20`


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