回答:
#V = 21#
说明:
#1 / V +(3V + 12)/(V ^ 2-5V)=(N-56)/(V ^ 2-5V)#
#1 / V +(3V + 12)/(V ^ 2-5V) - (N-56)/(V ^ 2-5V)= 0#
共同点是 ·V ^ = 2-5V V(V-5)#
#(V-5 + 3V + 12-(7V-56))/(V ^ 2-5V)= 0#
#(V-5 + 3V + 12-7v + 56)/(V ^ 2-5V)= 0#
#(V + 3V-7V-5 + 12 + 56)/(V ^ 2-5V)= 0#
#( - 3V + 63)/(V ^ 2-5V)= 0#
#-3V + 63 = 0#
#-3V = -63#
·V =( - 63)/( - 3)#
#V = 21#