In a transformer a sinusoidally varying voltage is applied to the primary coil of the transformer which give rise to a time varying flux which induce another sinusoidally varying EMF in the primary coil. The induced EMF has a 180° phase difference with the applied voltage and hence opposes it due to which the voltage across the coil should decrease leading to a decrease in the max value of the time varying flux. So the decrease in flux should then also decrease the max value of induced EMF.
How can the EMF across the primary then continue to be sinusoidal when the max value of sinusoidally varying out of phase induced EMF change with time? Shouldn't it be represented by a sinusoidal curve whose max value change with time?