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Using the distance formula :

AB  = sqrt  [ (5 - -1)^2 + ( 4 - - 1)^2 ]  =

sqrt [ 6^2  + 5^2]  =

sqrt [36 + 25]  =

sqrt [61]  ≈  7.8 units

What is the length of side AB in the following triangle rounded to the nearest tenth?

$\begin{array}{|rcll|} \hline AB &=& \sqrt{5^2+6^2} \\ &=& \sqrt{25+36} \\ &=& \sqrt{61} \\ &=& 7.81024967591 \\ &\approx& 7.81 \\ \hline \end{array}$