连接AR
∵AN=NC
∴SΔANR=SΔCNR=5 cm²SΔANB=SΔCNBSΔARB=SΔCRB
又∵4AM=BM
∴4SΔAMR=SΔBMR4SΔACM=SΔBCM
设SΔAMR=X,则:SΔBMR=4X, SΔCRB=5X
∵ 4SΔAMC=SΔBMC
4*(SΔAMR+SΔANR+SΔCNR)=SΔBMR+SΔCRB
4*(X+5+5)=4X+5X
4X+40=9X
X=8 cm²
SΔABC=SΔAMC+SΔBMC
=(SΔAMR+SΔANR+SΔCNR)+(SΔBMR+SΔCRB)
=8+5+5+4*8+5*8
=90 cm²