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Reds get off to a flying start
Alexander gave Reds the lead in the 33rd minute when a header from Hope Akpan hit him and deflected into the net from close range.
The striker made sure of the points in the 64th minute after he was brought down in the area and picked himself up to score from the spot.
New loan signing Jonathan Forte completed a perfect afternoon for Richie Barker's men when he came off the bench to nod home a third with ten minutes remaining.
The visitors were reduced to ten men late on when Ryan Duffy received a second yellow card.
Reds began brightly and Alexander could have given them the lead after just 37 seconds when he shot over the bar.
Billy Clarke hit the post with an effort from 20 yards and was also denied by a good block from goalkeeper Sam Slocombe.
Clarke was involved again as Reds made the breakthrough. He won a free kick on the left which Akpan headed across goal with Alexander getting the decisive touch.
Scunthorpe had two good chances to level before half-time with Bobby Grant denied by a fine interception from Mat Sadler and Dave Mirfin volleying wide from a corner.
Reds extended their lead 19 minutes after the interval. Mark Connolly's cross was headed back across goal by Clarke and the referee pointed to the spot as Tom Newey tangled with Alexander.
Alexander made no mistake with the penalty, sending it low to the goalkeeper's right.
Forte came on for his debut with 16 minutes to go and was celebrating a goal six minutes later. Scunthorpe stopped with a player from either side on the ground and Akpan sent a long ball over the top which Forte beat goalkeeper Slocombe to and nodded into the net.
Fellow loan signing Nicky Ajose came off the bench for his debut late on and the day got worse for Scunthorpe when Duffy got his marching orders.