Ta có tam giác ABC vuông tại A góc \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaecaaeaaca % WGbbGaamOqaiaadoeaaiaawkWaaiabg2da9iaaiodacaaIWaGaeyiS % aalaaa!3D74! \widehat {ABC} = 30^\circ\) và BC = a, suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaiaado % eacqGH9aqpdaWcaaqaaiaadggaaeaacaaIYaaaaiaacYcacaaMc8Ua % amyqaiaadkeacqGH9aqpdaWcaaqaaiaadggadaGcaaqaaiaaiodaaS % qabaaakeaacaaIYaaaaaaa!419C! AC = \frac{a}{2},\,AB = \frac{{a\sqrt 3 }}{2}\) .

Lại có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGPaVlaayk % W7daGabaabaeqabaWaaeWaaeaacaWGtbGaamyqaiaadkeaaiaawIca % caGLPaaacqGHLkIxdaqadaqaaiaadgeacaWGcbGaam4qaaGaayjkai % aawMcaaaqaaiaadoeacaWGbbGaeyyPI4LaamyqaiaadkeaaaGaay5E % aaGaaGPaVlaaykW7cqGHshI3caWGbbGaam4qaiabgwQiEnaabmaaba % Gaam4uaiaadgeacaWGcbaacaGLOaGaayzkaaaaaa!5513! \,\,\left\{ \begin{array}{l} \left( {SAB} \right) \bot \left( {ABC} \right)\\ CA \bot AB \end{array} \right.\,\, \Rightarrow AC \bot \left( {SAB} \right)\), suy ra tam giác SAC vuông tại A .

Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uaiaadg % eacqGH9aqpdaGcaaqaaiaadofacaWGdbWaaWbaaSqabeaacaaIYaaa % aOGaeyOeI0IaamyqaiaadoeadaahaaWcbeqaaiaaikdaaaaabeaaki % abg2da9maakaaabaGaamyyamaaCaaaleqabaGaaGOmaaaakiabgkHi % TmaabmaabaWaaSaaaeaacaWGHbaabaGaaGOmaaaaaiaawIcacaGLPa % aadaahaaWcbeqaaiaaikdaaaaabeaakiabg2da9maalaaabaGaamyy % amaakaaabaGaaG4maaWcbeaaaOqaaiaaikdaaaaaaa!4A4D! SA = \sqrt {S{C^2} - A{C^2}} = \sqrt {{a^2} - {{\left( {\frac{a}{2}} \right)}^2}} = \frac{{a\sqrt 3 }}{2}\)

Tam giác SAB có \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uaiaadg % eacqGH9aqpdaWcaaqaaiaadggadaGcaaqaaiaaiodaaSqabaaakeaa % caaIYaaaaiaacYcacaaMc8UaamyqaiaadkeacqGH9aqpdaWcaaqaai % aadggadaGcaaqaaiaaiodaaSqabaaakeaacaaIYaaaaiaacYcacaaM % b8UaaGPaVlaadofacaWGcbGaeyypa0Jaamyyaaaa!49DE! SA = \frac{{a\sqrt 3 }}{2},\,AB = \frac{{a\sqrt 3 }}{2},\,SB = a\) . Từ đó sử dụng công thức Hê-rông ta tính được .\(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBa % aaleaacaWGtbGaamyqaiaadkeaaeqaaOGaeyypa0ZaaSaaaeaacaWG % HbWaaWbaaSqabeaacaaIYaaaaOWaaOaaaeaacaaIYaaaleqaaaGcba % GaaGinaaaacqGHshI3caWGtbGaamisaiabg2da9maalaaabaGaaGOm % aiaadofadaWgaaWcbaGaam4uaiaadgeacaWGcbaabeaaaOqaaiaadg % eacaWGcbaaaiabg2da9maalaaabaGaamyyamaakaaabaGaaGOnaaWc % beaaaOqaaiaaiodaaaGaeyO0H4TaamOqaiaadIeacqGH9aqpdaWcaa % qaaiaadggadaGcaaqaaiaaiodaaSqabaaakeaacaaIZaaaaiabg2da % 9maalaaabaGaaGOmaiaadgeacaWGcbaabaGaaG4maaaaaaa!580F! {S_{SAB}} = \frac{{{a^2}\sqrt 2 }}{4} \Rightarrow SH = \frac{{2{S_{SAB}}}}{{AB}} = \frac{{a\sqrt 6 }}{3} \Rightarrow BH = \frac{{a\sqrt 3 }}{3} = \frac{{2AB}}{3}\)

Suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamizamaabm % aabaGaamisaiaacYcadaqadaqaaiaadofacaWGcbGaam4qaaGaayjk % aiaawMcaaaGaayjkaiaawMcaaiabg2da9maalaaabaGaaGOmaaqaai % aaiodaaaGaamizamaabmaabaGaamyqaiaacYcadaqadaqaaiaadofa % caWGcbGaam4qaaGaayjkaiaawMcaaaGaayjkaiaawMcaaiaac6caaa % a!48EC! d\left( {H,\left( {SBC} \right)} \right) = \frac{2}{3}d\left( {A,\left( {SBC} \right)} \right).\)Từ H kẻ \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiaadU % eacqGHLkIxcaWGcbGaam4qaaaa!3AD1! HK \bot BC\)

Kẻ \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiaadw % eacqGHLkIxcaWGtbGaam4saiabgkDiElaadIeacaWGfbGaeyyPI41a % aeWaaeaacaWGtbGaamOqaiaadoeaaiaawIcacaGLPaaaaaa!4479! HE \bot SK \Rightarrow HE \bot \left( {SBC} \right)\). Ta dễ tính được \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiaadU % eacqGH9aqpdaWcaaqaaiaadggadaGcaaqaaiaaiodaaSqabaaakeaa % caaI2aaaaiabgkDiElaadsgadaqadaqaaiaadIeacaGGSaWaaeWaae % aacaWGtbGaamOqaiaadoeaaiaawIcacaGLPaaaaiaawIcacaGLPaaa % cqGH9aqpdaWcaaqaaiaadggadaGcaaqaaiaaiAdaaSqabaaakeaaca % aI5aaaaiaac6caaaa!49C1! HK = \frac{{a\sqrt 3 }}{6} \Rightarrow d\left( {H,\left( {SBC} \right)} \right) = \frac{{a\sqrt 6 }}{9}.\)

Vậy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamizamaabm % aabaGaamyqaiaacYcadaqadaqaaiaadofacaWGcbGaam4qaaGaayjk % aiaawMcaaaGaayjkaiaawMcaaiabg2da9maalaaabaGaaG4maaqaai % aaikdaaaGaamizamaabmaabaGaamisaiaacYcadaqadaqaaiaadofa % caWGcbGaam4qaaGaayjkaiaawMcaaaGaayjkaiaawMcaaiabg2da9m % aalaaabaGaaG4maaqaaiaaikdaaaGaeyyXIC9aaSaaaeaacaWGHbWa % aOaaaeaacaaI2aaaleqaaaGcbaGaaGyoaaaacqGH9aqpdaWcaaqaai % aadggadaGcaaqaaiaaiAdaaSqabaaakeaacaaI2aaaaaaa!5352! d\left( {A,\left( {SBC} \right)} \right) = \frac{3}{2}d\left( {H,\left( {SBC} \right)} \right) = \frac{3}{2} \cdot \frac{{a\sqrt 6 }}{9} = \frac{{a\sqrt 6 }}{6}\)