Let ΔABC be an obtuse triangle with sides a, b, c, that lie
opposite the angles A, B, C; with radius r of inscribed circle and
radius R of circumscribed circle; p semiperimeter. For obtuse triangle you can use all known area formulas:
1.
; (the area is equal to half of product of two sides and sine of angle that lies between these sides);
2.
;
3.
(Heron's formula);
4.
;
5.
.
1.
2.
3.
4.
5.