trikampiu plotas, ju krastiniu ieskojimas

kokia formule taikyti norint apskaiciuoti trikampio ABD plota 6 pratime? :)

[tex]a^2 = c^2 - b^2[/tex](Pitagoro teorema)
[tex]a^2 = 17^2 - (10+5)^2;[/tex]
[tex] a^2 = 64;[/tex]
[tex] a = 8;[/tex]
[tex] S_{ADB} = S_{ABC} - S_{BCD};[/tex]
[tex] S_{ADB} = 10 * 8 / 2 - 5*8/2 = 40 - 20 = 20;[/tex]