a: Xét ΔAMB và ΔAMC có

AM chung

góc MAB=góc MAC

AB=AC
=>ΔAMB=ΔAMC

b: Xét ΔMAB và ΔMNC có

góc AMB=góc NMC

MB=MC

góc ABM=góc NCM

=>ΔMAB=ΔMNC

=>AB=NC

c: ΔMAB=ΔMNC

=>MA=MN

=>AM=1/2AN

a) 

A=(\(\dfrac{36}{12}\) - \(\dfrac{9}{12}\)+\(\dfrac{8}{12}\)) - ( \(\dfrac{24}{12}\) + \(\dfrac{16}{12}\) - \(\dfrac{18}{12}\) ) - ( \(\dfrac{12}{12}\) - \(\dfrac{21}{12}\) - \(\dfrac{54}{12}\))

= \(\dfrac{19}{12}\) - \(\dfrac{22}{12}\) - \(\dfrac{-63}{12}\)

= 5