Please help with formula 9 & 10 and then

1. In the space below, explain why the operating income calculated by the absorption method is not the same as that calculated by the variable cost method.

Data Section
Actual production in units	100,000
Sales in units	80,000
Sales price per unit	$30
Variable manufacturing costs per unit	$14
Variable selling costs per unit	$2
Fixed manufacturing costs	$315,000
Fixed selling expenses	$100,000
Answer Section
Income statement: Absorption costing
Sales		$2,400,000
Cost of goods sold:
Variable manufacturing costs	$1,400,000
Fixed manufacturing costs	315,000
Total goods available for sale	$1,715,000
Less ending inventory	343,000
Cost of goods sold		1,372,000
Gross profit		$1,028,000
Selling expenses:
Fixed selling expenses	$160,000
Variable selling expenses	100,000
Total selling expenses		260,000
Operating income		$768,000
Income statement: Variable costing
Sales		$2,400,000
Cost of goods sold:
Variable manufacturing costs	$1,400,000
Less ending inventory	280,000
Variable cost of goods sold		1,120,000
Manufacturing margin		$1,280,000
Variable selling expenses		FORMULA9
Contribution margin		$1,280,000
Fixed costs:
Fixed manufacturing costs	FORMULA10
Fixed selling expenses	100,000
Total fixed costs		100,000
Operating income		$1,180,000