$ a)\sqrt {6,{8^2} - 3,{2^2}}  = \sqrt {\left( {6,8 - 3,2} \right)\left( {6,8 + 3,2} \right)}  = \sqrt {3,6.10} \\  = \sqrt {36}  = 6\\ b)\sqrt {21,{8^2} - 18,{2^2}}  = \sqrt {\left( {21,8 - 18,2} \right)\left( {21,8 + 18,2} \right)} \\  = \sqrt {3,6.40}  = \sqrt {144}  = \sqrt {{{12}^2}}  = \sqrt {12} \\ c)\sqrt {117,{5^2} - 26,{5^2} - 1440} \\  = \sqrt {\left( {117,5 - 26,5} \right)\left( {117,5 + 26,5} \right) - 1440} \\  = \sqrt {91.144 - 1440}  = \sqrt {144.\left( {91 - 10} \right)}  = \sqrt {144.81} \\  = \sqrt {{{\left( {12.9} \right)}^2}}  = 108\\ d)\sqrt {146,{5^2} - 109,{5^2} + 27.256} \\  = \sqrt {\left( {146,5 - 109,5} \right)\left( {146,5 + 109,5} \right) + 27.256} \\  = \sqrt {37.256 + 27.256}  = \sqrt {256\left( {37 + 27} \right)}  = \sqrt {256.64} \\  = \sqrt {{{\left( {8.16} \right)}^2}}  = 16.8 = 128 $

`a) \sqrt{6,8^2-3,2^2}`

`=\sqrt{(6,8-3,2)(6,8+3,2)}`

`=\sqrt{3,6. 10}`

`=\sqrt{36}`

`=6`

`b) \sqrt{21,8^2-18,2^2}`

`=\sqrt{(21,8-18,2)(21,8+18,2)}`

`=\sqrt{3,6.40}`

`=\sqrt{144}`

`=12`

`c) \sqrt{117,5^2-26,5^2-1440}`

`=\sqrt{(117,5-26,5)(117,5+26,5)-1440}`

`=\sqrt{91.144-1440}`

`=\sqrt{144(91-10)}`

`=\sqrt{144.81}`

`=12.9`

`=108`

`d) \sqrt{146,5^2-109,5^2+27.256}`

`=\sqrt{(146,5-109,5)(146,5+109,5)+27.256}`

`=\sqrt{37.256+27.256}`

`=\sqrt{256(37+27)}`

`=\sqrt{256.64}`

`=16.8`

`=128`