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The measure of the vertex angle is 100°

Step-by-step explanation:

In Δ MNO

∵ Δ MNO is an isosceles triangle with base MO

∴ NM = NO

→ In an isosceles triangle, the base angles are equal in measures

∵ ∠M and ∠O are the base angles isosceles ΔMNO

∴ m∠M = m∠O

∵ m∠M = (3x + 10)°

∵ m∠O = (5x - 10)°

→ Equate their measures

∴ 5x - 10 = 3x + 10

→ Add 10 to both sides

∵ 5x - 10 + 10 = 3x + 10 + 10

∴ 5x = 3x + 20

→ Subtract 3x from both sides

∴ 5x - 3x = 3x - 3x + 20

∴ 2x = 20

→ Divide both sides by 2 to find x

∵ =

∴ x = 10

→ Substitute the value of x in the measures of ∠M and ∠O to find them

∵ m∠M = 3(10) + 10 = 30 + 10

∴ m∠M = 40°

∵ m∠O = 5(10) - 10 = 50 - 10

∴ m∠O = 40°

→ The sum of the measures of the interior angles of a Δ is 180°

∵ m∠M + m∠O + m∠N = 180°

∵ 40 + 40 + m∠N = 180

→ Add the like terms

∴ 80 + m∠N = 180

→ Subtract 80 from both sides

∴ 80 - 80 + m∠N = 180 - 80

∴ m∠N = 100°

∵ ∠N is the vertex angle of Δ MNO

∴ The measure of the vertex angle is 100°

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step-by-step explanation: