`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`