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In DeltaABC, a=7, b=5 and c=8, then find...

In `DeltaABC`, a=7, b=5 and c=8, then find the value of cos2B.

Text Solution

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cosB`=(a^(2)+c^(2)-b^(2))/(2ac)=(7^(2)+8^(2)-5^(2))/(2 xx 7 xx 8)`
`=(49+64-25)/(112)=88/112=11/14`
Now `cos2B=2cos^(2)B-1`
`=2 xx 11/14 xx 11/14-1`
`=121/98-1=23/98`
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