Triangular numbers

The first four triangular numbers are 1,3,6,10. What is the 10th triangular number?
Help: A triangular number or triangle number counts objects arranged in an equilateral triangle.

Correct answer:

t10 =  55

Step-by-step explanation:

t1=1 t2=t1+2=1+2=3 t3=t2+3=3+3=6 t4=t3+4=6+4=10 t5=t4+5=10+5=15 t6=t5+6=15+6=21 t7=t6+7=21+7=28 t8=t7+8=28+8=36 t9=t8+9=36+9=45 t10=t9+10=45+10=55



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