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You need to use the law of cosines to find a such that:
`a^2 = b^2 + c^2 - 2bc*cos A`
You need to substitute 10 for b,7 for c and 60^o for A such that:
`a^2 = 100 + 49 - 140 cos 60^o`
You need to substitute `1/2` for `cos 60^o` such that:
`a^2 = 149 - 70 =gt a^2 = 79`
`a = +-sqrt 79`
You need to keep the positive value for the length a, thus `a = sqrt 79.`
Hence, evaluating the length of side a yields `a = sqrt 79` .
Draw a height from point C to side c called CD.
The height CD=b*sinA=bsinA
The length AD=b*cosA=bcosA
The side a=sqrt(CD^2+DB^2)=sqrt((bsinA)^2+(c-bcosA)^2)
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